Reprinted from The Pacific Mineralogist, December 1938; semi-annual publication of the Los Angeles Mineralogical Society; Vol. V, No. 2; p.10-11, (36pp).
(Footnotes added)
© 1938 Los Angeles Mineralogical Society, Huntington Park, Calif.
TOURMALINE
By E. E. TOMPKINS
Among the most interesting and beautiful of gem-minerals are the highly colored varieties of tourmaline. The ordinary tourmalines are black or brown and are widely distributed in igneous and metamorphic rocks of various kinds. The light colored gem varieties are found chiefly in the pegmatite dikes in various parts of the world.
Notable locations for the occurrence of gem tourmaline are Madagascar, the Ural mountains, Brazil, Island of Elba, the gem gravels of Ceylon, and in the United States at Paris and Auburn Maine; Haddom Neck, Conn., and our own state of California at
Mesa Grande,
Pala,
Rincon and
Ramona, in San Diego County, also a few places in Riverside County. However, it is regrettable that at the present time none of the California areas are producing tourmaline.
The first recognition of these gem minerals in California apparently goes back as far as 1872 when a Mr. Hamilton obtained some very fine colored tourmaline in the southwest slope of Thomas mountain in Riverside county
[1].
The first discovery in San Diego county is thought to have been made about 1880, when some Indian children while playing in a camp near what is now Mesa Grande
[2], picked up an oddly shaped stone about three inches long and slightly thicker than an ordinary lead pencil; on cleaning it and rubbing it with a piece of hide it was found to be a beautiful blue color, partly clear, very similar to a sapphire. Subsequently other colored stones, some blue some pink, and others of a green color were picked up in the same vicinity by the Indians and cowboys but no one realized they had any actual value.
The first important development at Pala was in 1890
[3], a ledge of
lepidolite containing
rubellite (pink tourmaline) was traced for quite a distance. The rubellite crystals are clustered in radiating groups in the fine mica; they are not large and not clear, therefore, are not suitable for cutting into gems. Their rich rose red color on the lepidolite background made them excellent specimens for collectors, and have been placed in museums all over the world.
A number of mines around Pala have produced fine gem material. The Pala Chief
[4] was noted for its exceptionally large crystals. Some were as much as twelve inches in length and three inches across, of rich pink rubellite with an exterior coating of dark blue separated by a pale intervening zone.
Mesa Grande was also noted for large size and perfection of the crystals, many of them being doubly terminated. The two-color crystals were part red and part green, with contact being very strong, so that many were cut showing one-half of the gem red and the other half green. One gem of over fifty carats was cut from this type of material.
Quite a number of beautiful cat's-eye weighing up to 30 carats have been found, the colors varying from almost colorless to pale pink, rose, red, pale green, yellow-green and dark green. Mesa Grande was pre-eminent for the type of tourmaline. The Morgan collection in the American Museum of Natural History, New York, contains a fine series of crystals from this locality.
The colors of tourmaline vary somewhat in different world localities. Ceylon colors are mostly yellow, or yellowish-green, sometimes fine olive green. Some of the Madagascar tourmaline is of a fine brownish-red, almost as deep as a light garnet, and much clearer than most garnets. Brazilian tourmaline are usually green, but sometimes red.
In many localities several colors of tourmaline are usually found together, and it may be that a single crystal will be green in most of its length but red or pink-tipped. Some crystals have a pink core and a green exterior, or vice-versa. Some of the Maine tourmaline is of a slightly bluish-green tint that almost approaches emerald in color.
The California colors, as previously noted, are green, pink, red, blue and a few of the colorless variety,
achroite. The quality of some of the California tourmaline is not excelled by that from any other locality in the world.
The name tourmaline comes from turamali
[5], a name given to the early gems from Ceylon. Tourmaline is a very complex silicate containing aluminum, magnesium, iron, boron, lithium or other alkali with small amounts of water and fluorine. The system of crystallography is hexagonal, the hardness is 7.5, specific gravity 3.10, double refractive, refractive index 1.62-1.65.
A very curious property of tourmaline is its capability of developing a charge of pyro-electricity; when a crystal of tourmaline is changing in temperature, either warming or cooling, it develops a charge of positive electricity at one end and a negative charge at the other. This may be demonstrated by directing a mixture of red lead and sulphur through a fine sieve over a cooling crystal when one end will attract the red lead and the other the sulphur and they then show red and yellow.
Tourmaline are remarkable also for certain optical properties which render them incapable of being successfully imitated. Dichroism is very strong. In dark colored crystals (especially brown or black) the ordinary ray is completely absorbed, while the extraordinary is allowed to pass. Such a crystal, when viewed through the prism faces, will show some transmitted light and color, but when viewed through the basal plane it will be quite opaque. Two thin slices cut parallel to the vertical axis of such a crystal provide a simple polarizing apparatus. When the slices are in parallel position light is allowed to pass through, but when they are in crossed position the light is cut out. A suitable crystal placed between the crossed slices will show brilliant polarization colors as seen in the polarizing microscope. On account of this strong absorption of the ordinary ray, darker colored stones should be cut with the table facet parallel to the prism faces, while paler stones will show a better color when cut with the table parallel to the basal plane.
When pressure is applied in the direction of the polar axis of a crystal of tourmaline a charge of 'piezo-electricity' is developed. This property has an important technical application in detecting and registering small variations in pressure. In this direction tourmaline is much more useful for telling the depth of submarines than as a precious stone.
References:
"A Key to Precious Stones," by L. J. Spencer.
Bulletin No. 37, California Mining Bureau; "Gems, Jewelers Materials and Ornamental Stones," by George F. Kunz.
Footnotes:
| 1. | See the Columbia mine, first claimed in 1872 (under the General Mining Act of 1872), and reported by George F. Kunz in 1893, and the Riverside Press in 1897. |
| 2. | See the Himalaya mine, first claimed in 1898 (under the General Mining Act of 1872), and reported by George F. Kunz in 1900. |
| 3. | See the Alvarado mine, first claimed around 1872 (under the General Mining Act of 1872), and reported by William P. Blake in 1882, and the San Diego Union Tribune in 1885, and by Charles R. Orcutt in 1892. |
| 4. | The Pala Chief Gem Mining Company worked several localities in Pala, including the Tourmaline Queen mine and Pala Chief mine, both of which were promoted by Frank A. Salmons as the "Pala Chief Gem Mines". |
| 5. | Turamali is more commonly spelled Turmali. |
