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Isaac Lea on Inclusions in Gems & Minerals

Last Updated: 22nd Feb 2008

By Daniel Russell

Isaac Lea on Inclusions in Gems and Minerals

by Daniel E Russell

Inclusions in crystals provide valuable information to mineralogists, gemologists and geologists. In addition to offering vital data on the temperature and pressure at which the host crystal was formed, and serve as a chemical time capsule preserving a microscopic sample of the environment in which the crystal grew. Inclusions can provide a valuable clue to the identity of gemstones.
Isaac Lea circa 1885
Click for larger image

Between 1869 and 1877, US naturalist Isaac Lea of Philadelphia published three seminal papers on his observations of inclusions in gems and minerals. Born in Delaware in 1792, Leas achieved prominence as on of the nation’s leading malacologists. An avid collector of minerals and gems, Lea was not the first to write about inclusions. Scientists have been fascinated with inclusions at least as early as Robert Boyle, who in his 1672 “Essay on the Virtue of Gems” wrote of a “piece of Chrystal, in the midst of which was a drop of Water…” The brilliant British mineralogist Henry Clifton Sorby had published an extremely comprehensive tract on fluid and gas inclusions in minerals and rocks in 1858. His research documented the morphology of different types of inclusions, and he offered formulae to calculate the pressure and the temperature at which they formed.

Lea took a slightly different approach. Instead of the generalities of inclusion morphology and the chemistry and physics of their contents, he was more interested in a species-driven approach, eagerly collecting data on what types of inclusions characterized corundum, garnet-group minerals, topaz, spinel, and quartz. Lea’s work concentrated mainly on gems, in part because of his fascination with gems as well as the simple fact that the polished surfaces of facetted stones provided a more perfect window through which to examine inclusions.

Today, inclusions in minerals and gems are classified as:

Protogenetic inclusions are inclusions of minerals that were already present before the host mineral formed. Common examples are rutile or tourmaline in quartz.

Syngenetic inclusions are inclusions that were formed at the same time as the host mineral. These can be solid (other minerals that were deposited on the growing surface of the host mineral, then “swallowed up” by continued growth) or as cavities or “bubbles” of gas or liquid encapsulated in the host mineral. The shape of the cavities can range from spheres to weird, amoeboid shapes, and sometimes even mimics the morphology of the host mineral (the last being commonly called “negative crystals”) Liquids can include water (frequently as saline brines), carbon dioxde, and petroleum (and, sometimes, two immiscible liquids side-by-side). Gases can include air, carbon dioxide, and methane (in Lea’s day, an effort to introduce “Brewsterlinite” and “Cryptolinite” as names for liquefied gas inclusions was introduced by, happily, failed). Inclusions can consist of combinations of a gas and liquid, or solid and liquid (typical “two phase inclusions”) or a gas, liquid and solid (called a “three phase inclusions”).

Epigenetic inclusions are inclusions that were formed after the host crystals was formed. A typical epigenetic inclusion are exsolutions of fine rutile needles in corundum and garnet (which is responsible for creating asterism or “star” effects in those minerals and others). “Asteria” was the term used in Lea’s time for gems which exhibit asterism… Exsolution creates the alternating layers of orthoclase and albite which give “moonstone” its interesting optical qualities. Inclusions resulting from the recrystallization (‘healing”) of a fracture in a host mineral, trapping gas or liquid inside, are also epigenetic. {for a good volume on the mineralogical and geological implications of inclusions, see Roedder, 1984; for three full volumes of information on inclusions in gem materials, see Gübelin and Koivula, 2004, 2006, and 2008)

By the time of his death in 1886 (at age 95), he had accumulated a large collection of mineral specimens and seashells (which he willed to the Smithsonian Institute) and gemstones (originally bequeathed to his daughter Francis Lea Chamberlain, who subsequently donated it to the Smithsonian with an endowment for its maintenance). Of the collection, George F Kunz noted:

A remarkable and most interesting collection was made by Dr. Isaac Lea, of Philadelphia, who died in December, 1886. This eminent scientist for the last thirty years of his long life devoted much time to the study of microscopic inclusions in gems and minerals. The cabinet bequeathed by him to his daughter contains thousands of specimens of rubies, sapphires chrysoberyls, tourmalines, garnets, quartz, and other stones, each specimen labelled, and generally accompanied by a draw­ing, showing the interesting inclusions. His extensive bibliogra­phy includes several papers on inclusions in precious stones.


Notes On Microscopic Crystals Included In Some Minerals
By Isaac Lea

Proceedings Academy Of Natural Sciences Of Philadelphia. 1869

During some years past I have given much attention to the examination of minerals under the microscope, and some of the observations were published in the Proceedings of the Academy in 1866.

In none of these have I found any mention of these inclusions. But in that excellent work "Repertoire D'Optique Moderne," by M. l'Abbé Moigno, where he treats of optical mineralogy, I found that he states M. Babinet to have examined “star garnets” (Granats asteriques) some with four and some with six branches. He says that the star garnets with four branches are not very rare — 20 to 30 in 1000 to 1200 — but that the star of six rays he found only one in 6000 specimens. Whether the filaments or fibers, as M. Babinet calls the asteroid reflections, are the same as the acicular crystals observed by me I cannot say, but certainly these latter are more common so far as my observation has extended, and I have observed no asterisms whatever. About a year since, in the examination of a thin fractured piece of a large garnet from North Carolina, I was surprised to observe a number of very minute acicular crystals, which generally took two or three directions. This induced me to examine more closely into the varieties of garnets which were accessible to me, and supposing these crystals might have been observed by others, I referred to the principal works on mineralogy which have been published in France, Germany and in this country.

In 154 specimens of Bohemian polished garnets I found 48 with acicular crystals! This far exceeds the proportion stated by M. Babinet. In the precious garnet from Green's Creek, Delaware Co., Penn., (uncut specimens), I found in the close examination of 310 specimens that 75 were possessed of acicular crystals, being nearly 25 per cent — a very much larger percentage than mentioned by M. Babinet. Of the Brazilian pyrope I examined 40 specimens. They were very pure and free from spots and cavities. I could not find a single acicular crystal in any one of them. In Essonite I found no acicular crystals in the few specimens which I had it in my power to examine, nor in grossularite, ouvarovite, colophonite or massive magnesium garnet.

Cinnamon-stone [Grossular] from Dixon's, near Wilmington, Del., was carefully examined in nearly 60 specimens, none of which showed any trace of acicular crystalization.

Spinelle ruby, of which I examined 28 specimens, produced no microscopic crystalized forms.

It will be difficult to ascertain what composes these microscopic crystals in garnets, but they may prove to be rutile when chemical analysis shall be able to resolve the difficulty.

Further Notes on Microscopic Crystals in some of the Gems, &c
By Isaac Lea

Proceedings Academy Of Natural Sciences Of Philadelphia. 1869

In a paper which I recently read to the Academy, I mentioned having found acicular crystals in Precious Garnets. Since then I have had the opportunity of examining a number of cut specimens of Sapphire in the forms of Asteria, Catseye, &c. I have also examined many specimens of Cinnamon Stone from Ceylon, brought by Dr. Ruschenberger, of the United States Navy, also, among others, a very fine specimen of bluish Sapphire, in the collection of Prof. Leidy.

Having made microscopic drawings of these and other species, having included microscopic crystals, I propose to present them with as nearly correct illustrations as possible.

The whole subject of microscopic mineralogy has been of great interest to me, and I hope these short notes may induce some student to pursue the subject to a greater extent than I have had it in my power to do. It cannot fail that, with the use of the numerous admirable microscopes now made in this country, working with so much more facility than with those we have been accustomed to from abroad, observers may continue to bring to our knowledge much that has been heretofore unknown and very little suspected in this branch of science.

In my former paper I stated the proportional number among Bohemian Garnet which I found to contain microscopic crystals. I now propose to give descriptions and figures of the appearance of these crystaline forms, and with this view I have made drawings of their apparent forms under a power of about 100 diameters.

Sapphire – A very remarkably beautiful Asteriated bluish Sapphire, procured by
Dr. Ruschenberger when in Ceylon, presented to the naked eye the six rays which in the sun were sharp and of great beauty. The specimen being set as a gem of luxury, I could not get a view by transmitted light, but by reflected light, with great care, the exceedingly minute crystals were distinctly seen. They are very short, of pearly lustre, at three different equal angles, thus producing the bands which form the rays in three directions of 60° each. The reflection from the sides of these minute crystals cause, of course, the asterism of six rays over any point of the curved polished surface of the specimen. These rays are formed on the same principal precisely as the asterism in Phlogopite, which I have mentioned elsewhere.

Lea’s Figues of Inclusions
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Fig. 1 represents the delicate, numerous, minute crystals in the beautiful Asteria referred to above belonging to Dr. Ruschenberger. The acicular crystals are so small that it was with great difficulty I obtained their position as here represented.

The variety of Sapphire (Corundum) which goes under the name of Catseye, has irregular coarse striae, which have the appearance of being Asbestus as is generally supposed. In this gem there is a single band which varies according to the position it may be placed in, and by no means has the beauty of the asteriated Sapphire. Several of these are now before me which came from Ceylon.

Fig. 2 represents the crystals which I observed in a fine small bluish Sapphire, in Prof. Leidy’s fine collection of gems. The cuneiform or arrow-headed crystals are very extraordinary, and they may be simply twin crystals of some substance of which at present we can have no perfect-idea. They remind us in their form of Selenite crystals, such as are found in the Paris Basin, and at once we recognise the similarity to the cuneiform character stamped on the bricks of Babylon, and cut in the alabaster monuments of Nineveh. The group which I have drawn represents six of these cuneiform crystals, and six acicular crystals. Of the former six, four had a bluish tinge and two were pinkish. The acicular crystals were disposed to take three different directions, parallel to the primatic (sic - prismatic) hexagon sides of Corundum. Both sets of these crystals are enlarged to about 200 diameters, for the purpose of giving distinctly their very singular form.

Specimens of Garnet examined from all localities obtainable, presented very different aspects. When crystals 'were found in them they always proved to be acicular in form, but by no means similarly regular or of the same length, direction, or of the same size.

Fig. 3. A Bohemian cut Garnet presented three sets of numerous, thickly set, parallel, acicular crystals, which crossed at an angle of 120°, forming a very regular lattice-work appearance.

Fig. 4. A Bohemian cut Garnet presented only two sets of acicular crystals, which were usually at right angles, but some were inclined from perpendicularity and they were not so long as those of figure 3.

Fig. 5. A Bohemian cut Garnet presented a very different set of crystals. They were generally short, comparatively, and pointed in every possible direction.

Fig. 6. Garnet from Ceylon—Cinnamon-stone—fractured portions, not cut and polished. The acicular crystals were much shorter, rather thicker and much more bluntly terminated than in Fig. 5. They are placed at all angles. Ten specimens only in 80 examined had any thing like crystals, while all had irregular rifts or cavities within.

Fig. 7. Precious Garnet = Pyrope from Green’s Mill, Delaware Co., Penn., presented acicular crystals somewhat like Bohemian Garnet, fig. 3, but the three sets, while they take the same three directions, are shorter and left interspaces as shown in the figure.

Fig. 8. Garnet from North Carolina. A thin fracture from a compact garnet of large size, perhaps two inches in diameter. The acicular crystals are not vеry numerous—they are thin and not continuons. Connected with these are a few dark crystals. These take no particular direction like the others, but seem to be interspersed throughout.

Fig. 9. Labradorite. This specimen is a email polished one from Ceylon, and belongs to Dr. Ruschenberger. Besides the usual play of pavonine colors in Labradorite, I have found in all the specimens I have examined from various other localities, very minute reflecting crystals like those in Sunstone, and which are no doubt the same, but differing in size, being smaller so far as I have observed. The microscopic forms as figured will be observed to consist of two sets apparently distinct. The larger are rather irregular and black. The thinner are rather shorter and more delicate. These are not the reflection of the plates of Göethite, they are the black crystals which are usually in dark Feldspar.

Fig. 10. Black Feldspar. A small specimen of black Feldspar, translucent in thin pieces, from Chester Co., Penn., presented quite a different appearance from Labradorite in its minute, black included crystals. They are very numerous, very short, opake black, and irregular in form. They are closely set and irregular in their direction. There were no reflections from any of these included crystals.

Fig. 11. Barite, from Antwerp, Jefferson Co., New York, a represents some opake crystals observed in a small prismatic crystal. They cannot be, I think, rifts, and yet they are evidently without planes. b represents singular impressions on the surface of one of the prismatic planes, and their singular form, like the common horse-shoe magnet, induces me to call attention to them.

Fig. 12. Amethyst. A specimen from Thunder Bay, Lake Superior, presents very remarkable globules, some of an orange-yellow and some of a dark-green. These are very visible to the naked eye, and in the figure they are not very greatly magnified. They vary somewhat in size, and the orange-colored ones are most numerous in the specimen before me. There is a cloudiness in these yellow globules and a few are not completely spherical, presenting a cup-shaped form. To the naked eye the green globules appear to be black, but under the microscope they are evidently dark green. The composition of the two sets are no doubt the same, and the color probably depends on their being in a different state of oxidation. In a few cases I observed the two colors in the same globule. In another specimen from the same locality I found the globules to be much smaller and the green ones to prevail.

Fig. 13. An Asteriated Sapphire, also belonging to Dr. Le Conte, of an obtuse conical form, and of unusual beauty, presented very remarkable microscopic crystals of a white silken hue. The larger of two sets were generally, though not always, cuneate and lay in three directions, differing somewhat in size. In the smaller set the crystals are very minute, having the same pure white, silken appearance. These fill up the interstices of the larger crystals.

A Sapphire of large size and peculiar beauty, in the possession of Dr. Le Conte, presented a few distant, white silk-like lines, running in one direction, and parallel to each other. It is of unusual brilliancy and fine color and is thirteen-twentieths by eleven-twentieths of an inch in size.

Fig. 14. A Pyrope from New Mexico, in which the microscopic crystals differ from any of the many Garnets I have examined. In other specimens from this locality — of which I have examined twenty in the collections of Prof. Frazer and Dr. Le Conte — acicular crystals alone were found. In this specimen the crystals are much larger, less in number and of an entirely different character. Some are geniculate and transparent, while some are dark or semi-transparent. A very short and rather thick crystal seems to present three sides of an hexagonal prism. These New Mexican Pyropes are of uncommon beauty and perfection. This specimen is in the collection of Prof. Frazer. His other seven specimens have acicular crystals. Of Dr. Le Conte-s twelve specimens, six had acicular crystals, and six presented no appearance of inclusions. When the acicular crystals are examined in the direct rays of the sun at right angles to their axis, they reflect all the spectral colors in a very beautiful manner.

A small brilliant Ruby, which has the appearance of being oriental, but which may be a Spinel Ruby, was found to be very full of long acicular crystals which were observed to be in all directions, and were to all appearances the same as observed in Precious Garnets. A larger specimen has the same kind of acicular crystals, but in this specimen these crystals take generally two directions and are oblique to each other.

Two out of four other very beautiful small Oriental Rubies = Sapphire were found to have very minute acicular crystals. In one of them these crystals were in three directions ; in the other they were in two directions. Both these gave that peculiar changable band observed in the “Catseye” Sapphires. All these rubies were cut as brilliants and were of great beauty. It is apparent that the microscopic crystals in the various minerals above described, cannot all be of the same substance. Their forms and appearance forbid that, and chemical analysis will never probably reach, with any degree of satisfaction, their ultimate constituents. Spectral analysis may, however, be able to give us some results when properly applied, which may in sоme measure satisfy us in regard to the composition of these interesting included microscopic crystals.

Notes on “Inclusions” in Gems, &c.
By Isaac Lea

Proceedings Academy Of Natural Sciences Of Philadelphia. 1877

In a communication on microscopic crystals contained in gems, which the Academy of Sciences of Philadelphia did me the favour to publish in its “Proceedings”' a few years since, I gave some figures of these crystals, which I have frequently since verified. I then observed that, beside these intercrystalline forms, there were in most gems cavities frequently so numerous that they amounted to tens of thousands.

Since the period of the publication of my paper, I have made very large additions to my cabinet of gems, and particularly those of the Corundum group, sapphires, rubies, and the so-called Oriental topaz, Oriental amethyst, Asteria, &c. In the numerous fine blue sapphires of my collection, I have rarely explored one without finding numerous cavities, and ordinarily also finding the beautiful microscopic acicular crystals, which, when the specimen is cut cabochon, cause the three bands, and these by crossing form the star in Asteria. The cuneate microscopic crystals are also quite common. Cavities, with or without the fluids, are so frequent in crystals, from the soft calcite to the hard corundum, that little may be said as to their occurrence, as they are so common.

Cavities in quartz crystals enclosing fluids have been observed by the older mineralogists, but the kind of fluid, and gas or air, was not ascertained by them. Sir Humphry Davy, in 1822, investigated the contents of these cavities, and found them generally pure water. The gas-bubbles were sometimes found to be "azote." [“azote” was the 19th Century term for nitrogen gas] Sir David Brewster, in 1823, published a memoir of great research and value. He first had his attention called to the examination of fluid in cavities by the explosion of a crystal of topaz when heating it. He found cavities and air-bubbles in nearly twenty different substances, and these inclusions were carefully examined by him. In some of these cavities he observed two fluids and crystals, and these are figured in his plates. Subsequently, Mr. Sorby published a long and admirable paper on fluid cavities and crystals in minerals, with numerous and interesting figures. He considered that the cubic crystals were probably chloride of sodium. In his investigation he proved, by forming artificial crystals, that, in a natural state, the fluid cavities, with their " inclusions," must have been formed by aqueo-igneous forces. He gives a figure of fluid in mica, but I have never seen any in that mineral, although many hundreds have passed under my microscope in looking after crystals of magnetite, &c. Mr. Sorby also published a paper on cavities in quartz in the ”Phil. Mag.” vol. xv. p. 153; also with Mr. Butler in “Proc. Roy. Soc. London,” vol. xvii. p. 299. Kirkel on “Microscopic Minerals,” Neues Jahrbuch, 1870, p. 80, mentions bubbles and cubic crystals in quartz. He found iron glance and fluid in Elasolite = Nephelite. In emery, from Naxos, he found fluid in cavities. In 1872, Mr. Sang published an account of water in cavities of calcite.

Very recently, Professor Hartley, King's College, London, has published a very able paper on the subject of the fluid in quartz, &c. He says that Simmler in 1858, offering an interpretation of Brewster's observations, concluded that the expansible liquid was carbon dioxide. Professor Hartley states that in many cases the liquid in quartz is water, but that in some cases he found the two fluids; and his very satisfactory and careful experiments show conclusively that the most volatile of the two fluids is carbonic dioxide. He found in every experiment that the fluid disappeared when exposed to 31° C., and reappeared on cooling. Professor Hartley accords with Mr. Sorby in his reasoning that these two fluids Professor Dana without any analysis has called Brewsterlinite and Cryptolinite.

Calcite– has been found to contain nearly a quart of this fluid, but it is not as common to be found in small cavities as it is in quartz at the time of its assuming the solid state, the solution endured a high temperature.

Fluorite– Cavities in this mineral are rarely found, but they are sometimes seen with fluid and air-bubbles.

Feldspar Group– In a former paper I gave the result of the examination of many specimens of various species. Since then I have examined numerous specimens of Labradorite, and found no cavities, but the black crystals were very numerous. In the moonstone of this country I have not observed cavities or crystals; but in two specimens, out of about one hundred from Ceylon, I have seen a series of very regular quadrate cavities or crystals which do not appear to have any fluid.

Apatite– I have never observed cavities in this mineral, but I have not given it much attention in microscopic examinations.

Tourmaline– This interesting mineral is found beautifully crystallized, and of almost all colours, white, brown, green, red, black, &c. The finest are found at Mount Mica, near Paris, Maine. Some of these specimens have small internal elongate crystals, which are terminated. A red specimen (Rubellite) in my collection has many irregular cavities. One green one from Ceylon has cavities with fluid, and another has very minute black acicular crystals in one direction. In brown crystals from Lower Dianburg, Carinthia, there are rough objects in the interior, evidently another mineral enclosed, which do not require the microscope to detect them.

Cyanite– Of the white and the blue varieties I have not observed any well-defined cavities or crystals; but in the grey-bladed cyanite, found at Cope's Mills, near West Chester, Pennsylvania, there are always, I believe, small black masses which do not take a regular form, but are usually elongate. These may easily be detected by splitting a crystal along its eminent cleavage, and examining the cleavage face with a lens of small power; but a higher power is preferable.

Quartz– takes upon itself many colours. In it are found cavities in very great numbers, particularly in the clear fine crystals. Those which exist in such an abundance in Herkimer County, New York, and which are so limpid, and finely and doubly terminated, are sometimes furnished with thousands of cavities, even in small specimens, and these are of many various forms, frequently containing fluid. In some cases the fluid may be seen to move by the unaided eye. In these Herkimer crystals, carbon in the form of anthracite is of very common occurrence, and in one of my specimens a small portion moves in the fluid of a cavity. These cavities often exist in an entire sheet, almost across the prism of a crystal.

In smoky quartz these cavities are much rarer, as also in amethyst and wine-colour and green quartz. The amethyst is frequently penetrated with crystals of rutile, and these are often very large, sometimes one to four inches long. The Chester County specimens usually have numerous curved filamentous crystals, easily detected with a common lens. In Way's Feldspar Quarry, near Dixon's, Delaware, there is a very peculiar form of quartz, which is nearly transparent, but somewhat clouded. The fragments of all sizes, from that of a pin's head to that of a small walnut, are enclosed in a mass of Deweylite. These fractured pieces are of indefinite forms. They are evidently cryptocrystalline, and look as if they may have been heated and suddenly cooled, and thus fractured. When these pieces are subjected to a high power, there may be detected in them very minute oval cavities in great numbers, and the major axes usually placed in one direction I have never seen cavities in milky quartz or blue quartz. Sir David Brewster found many cavities in rock crystal from Quebec with “water and mineral oil”

Topaz– In the various beautiful crystals which this mineral presents, there are frequently found cavities with fluid, and sometimes in this fluid may be seen the cuboid crystals described by Sir David Brewster. He found a single fluid in some cavities, and in others two fluids with “air-bubbles.” He says the fluid does not expand with heat. The Saxony transparent white crystals sometimes have cavities, as well as those of pale wine-colour. The Brazilian gold-yellow specimens have these cavities very frequently. The clear pinkish are more free from them. I have never observed any microscopic acicular crystals in topaz.

Emerald, Aquamarine, and Beryl– constitutionally the same - differ very much in regard to their possession of cavities and their commercial value. So far as I have been able to examine fine specimens of emerald, it is rare to see one without cavities. One which I have, of very fine colour, has many cavities of various forms, in which are included a fluid enveloping generally two perfect cubic crystals of an unknown mineral. In all cases in this specimen, the second crystal is much the smaller.

In aquamarine, cavities are not frequent, and in beryl I have found many cavities, and detected them only in a specimen from Unionville, Penn. In this there is a biangular cavity with a small cubic crystal at an inner angle. Throughout the mass there are small suboval cavities.

Garnet–– As a precious stone this is by no means rare, but it is lustrous, and of a fine colour. Cavities and microscopic crystals are very common in this gem. The cavities are usually irregular and rough, and never to my knowledge have fluid. On a polished surface of a piece of garnet from North Carolina, nearly an inch long, the reflexion of these crystals covered the whole surface with prismatic colours.

Cinnamon Stone– This beautiful variety of garnet, from Ceylon, as far as I have been able to observe it, and I have some twenty cut specimens, and numerous rolled pieces, has irregular cavities and some crystals, as I have stated in a former paper.

Zircon– With its high refractory power, this is used frequently as a gem, and sometimes sold as a diamond when white and perfectly transparent. One of the numerous specimens which I have examined has cavities and microscopic crystals, and a specimen from Ceylon has remarkable dark brown, elongate, fusiform spots, with numerous dotted ones intervening.

Chrysoberyl– The few specimens I have of this beautiful gem have neither cavities nor microscopic crystals, but Brewster observed “strata of cavities and both the fluids.”

Chrysolite = Olivine– In some of my specimens I have observed small cavities with fluid. Brewster met with them containing “fluid and bubbles of air.”

Spinel– This gem occurs of several colours. The Spinel ruby, so called, sometimes is very close in colour to the true ruby, but it has not by any means the depth nor brilliancy of the true ruby. In a pale-green specimen of great beauty which I have received recently from Ceylon, I have not been able to detect cavities or crystals. In my former papers I have expressed uncertainty in this matter.

Iolite– This gem is inferior in hardness, colour, and specific gravity to sapphire, but is valued for its peculiar change of colour, being dichroic. One of my specimens is without any inclusions. The other is filled with blue four-sided prismatic crystals, which are long, and enclosed in a nearly white subtransparent mass. These crystals are sometimes broken and their parts prolonged in the mass, and they are all lying in nearly the same direction.

Turquoise– with its peculiar and agreeable blue, is never transparent, and neither cavities nor microscopic crystals are found in it.

Opal– This exquisite gem, which displays such brilliant colours, is very highly valued. It is but little harder than glass, and is indeed considered as volcanic glass. Its remarkable flashes of colour are attributed to fissures, in accordance with the theory of Newton's coloured rings. I have never been able to detect either cavities or minute crystals in this beautiful gem, except in two cases. One of my specimens has a brown, terminated crystal, a six-sided prism of an unknown substance, about one-fifth of an inch long, and terminated by a single oblique plane; the other has several smaller ones.

Lapis-lazuli– This was used by the ancients as a favourite gem, but it is not now valued as such. I have not been able to detect cavities or minute crystals in any specimen in my possession.

Corundum– This very interesting mineral, when in perfect transparent crystals, is highly valued as a gem, under the name of sapphire, ruby, &c., according to colour. When yellow, it is called Oriental topaz; when purple, Oriental amethyst. When purely white it is sometimes sold as a diamond. In this country we have two localities only of corundum where any large quantity has been found, that of Chester County, Pennsylvania, and Franklin County, North Carolina. From the mines in Chester County, several hundred tons have been taken, but no transparent crystals. Some opaque ones are bluish and some pinkish. The North Carolina locality has produced some very large crystals, and numerous small ones. Of the latter there have been found many quite pure and transparent, and these are sometimes blue and sometimes red. But none of them yet found are of value as gems. The fine sapphires and rubies are chiefly from Ceylon, and they form some of the most beautiful objects in nature. I have many of these in the form of worn pebbles, and some in fine hexagonal form, as well as hundreds of cut specimens. I have examined carefully more than one thousand specimens, with a view to discover whatever “inclusions” they might possess. In a communication to the Academy I described and figured some microscopic crystals in these and other gems. Since then I have added a very large number to my collection, and among these several hundred large and small transparent crystals. In a careful microscopic examination of these, I found a large number which contain cavities and minute crystals, the former sometimes scattered irregularly through the mass, and sometimes forming a sheet or film. These cavities are of all forms, but usually subelliptical; sometimes tubular, and these tubes frequently anastomose in a very beautiful manner. These cavities are so numerous that they frequently give a cloudiness to the specimen, which is less valuable as a gem, but most interesting in a scientific point of view. In some specimens these cavities exist by tens of thousands, and Sir David Brewster stated that in a specimen under his observation there were about 37,000 of these cavities. I am sure that in one of my large cut specimens there must be more than double that number. It is a very common thing to see hundreds at a time of these cavities in the Ceylon specimens, partly filled with the fluids previously alluded to in these notes. But it is quite rare that they are found in the specimens from North Carolina. Still I have seen them in the transparent small fragments of deep blue crystals, and sometimes in the transparent light-coloured ones. In one specimen of the latter I discovered some most interesting cavities, which contained, beside the fluid, each a single cubic crystal. I had never observed an included crystal in any cavity in the numerous Ceylon specimens which I have examined. These cubic crystals have the exact form and appearance of those in the emerald described herein.

In some crystals of corundum there is a strong bronze reflexion, and this is the case with some of the large hexagonal crystals which were imported by Mr. S. S. White from India for commercial purposes, and which he distributed with so much liberality to our mineralogists. These bronze crystals have also been found at the Black Horse and Village Green localities in Delaware County, Pennsylvania. When examined with a good power, these bronze reflexions are at once seen to be caused by minute acicular crystals, and these may sometimes be seen in bunches.

In regard to the microscopic crystals in sapphire, having described and figured them in the papers before alluded to, I have little to add now. Further observation has confirmed what I then stated regarding the radii of Asteria. Very recently I have received a number of these Asteria of various colours, blue, purple, white, red, and dove colour; several three-quarters of an inch in diameter. The red and purple specimens are of peculiar beauty, and when examined in the sun, or any strong light, they both exhibit the microscopic acicular crystals with peculiar beauty, displayed as they are in hexagonal form, and reflecting the spectral colours. The ruby Asteria is certainly among the most beautiful objects in nature, and the purple are very little less so. A pale ruby, “Rubicelle,” which I lately received from my friend Hugh Nevill, Esq., Ceylon, about three carats, is a most interesting and beautiful gem. It has the depth and brilliancy almost of the diamond. It is nearly of a rose colour, and is perfectly transparent. It is cut with a top table and not entirely symmetrical. Its refractive power is unusually great. Yet when this brilliant transparent gem is examined with a high power and strong light, the whole mass may be seen to be filled with long acicular crystals in three directions, parallel to the prismatic planes, and interspersed are numbers of very minute and delicate cuneiform crystals. It has also a small cloud of exceedingly small cavities.

During the examination, about two years since, of some hundreds of small crystals of sapphire, perfectly transparent to dark blue, I discovered one which had very singular plumose impressions on the planes of the prism. This induced me to examine carefully
all those which I subsequently procured, and I have now over a dozen specimens which exhibit this very singular character. I am entirely at a loss to discover the cause of this form of minute impressions on so hard a substance. It evidently has been formed
by some collateral mineral substance, against which the molecules in crystallization have been arranged. Another remarkable specimen may be mentioned here, which has small cavities and minute microscopic crystals. It is of a pale yellow or straw colour, and of a depth and brilliancy scarcely exceeded by the diamond.

Diamond – The hardest of all substances stands first among gems. It has not, however, much interest to the microscopist, as no cavities with fluid have been, so far as known, observed; nor has it included crystals of foreign substances. They are often very imperfect, containing rifts and discolorations. Some of my specimens have beautiful triangular impressions on the surface of the planes. My friend Dr. Hamlin, of Bangor, Maine, is engaged on an extended work on the diamond. Such a work is much needed, and I know no one as capable as he to accomplish it. This gem sometimes occurs of various colours. In my cabinet I have six different colours.


Roedder, Edwin
Fluid Inclusions Review in Mineralogy Vol 12, Mineralogical Society of America. 1984

Boyle, Robert Essays On The Origin And Virtue Gems
London, 1672

Gübelin, Eduard and Koivula, John
Photoatlas of Inclusions in Gemstones, Volume 1 Opinio Verlag, Basel 2004

Gübelin, Eduard and Koivula, John
Photoatlas of Inclusions in Gemstones, Volume 2 Opinio Verlag, Basel 2006

Gübelin, Eduard and Koivula, John
Photoatlas of Inclusions in Gemstones, Volume 3 Opinio Verlag, Basel 2008

Lea, Isaac (1869a) “Notes On Microscopic Crystals Included In Some Minerals”
Proceedings Academy Of Natural Sciences Of Philadelphia. 1869 p.4

Lea, Isaac (1869b) “Further Notes on Microscopic Crystals in some of the Gems, &c”
Proceedings Academy Of Natural Sciences Of Philadelphia. 1869 pp. 119-121

Lea, Isaac (1877) “Further Notes on “Inclusions” in Gems, &c
Proceedings Academy Of Natural Sciences Of Philadelphia. 1877 pp. 98-107
Also in:
The Monthly Microscopical Journal: Transactions Of The Royal Microscopical Society
Volume XVII. 1877 pp 198-205

Roedder, Edwin
Fluid Inclusions Review in Mineralogy Vol 12, Mineralogical Society of America. 1984

Completed: 21 February 2008

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