Mindat Logo

Amethyst Specimens

Last Updated: 7th Jun 2013

Amethyst Specimens

Everything you always wanted to know, and needed to know about amethyst specimens, but were afraid to ask.
© Rock Currier June 1997

Amethyst, its name, some history, and why it has its beautiful color.
The name amethyst comes from the Greek “α-μεθηστos” which means “not drunk”. Roman emperors are said to have had cups/goblets made from amethyst in the belief that the cup would counteract any poison and keep them from getting drunk. Although as far as I know none of the ancient sources of amethyst ever produced single crystals of amethyst sufficiently large from which cups could be made; if a few large crystals of amethyst were found, they must have been very rare creatures indeed. I don't know of a single amethyst cup that has been preserved from antiquity. I think this tale of ancient amethyst cups may be mostly a myth. The ancients thought amethyst was different from quartz. They thought quartz was water that had been frozen so hard that it would never melt. Amethyst is a variety of quartz and has basically the same chemical composition, atomic structure, and physical properties of quartz: (hardness 7, specific gravity of 2.65 g/cc etc.) Books on geology and mineralogy usually list it under quartz. The bible mentions amethyst as one of the twelve stones in the breastplate of the High Priest. Each stone and thus represented one of the 12 tribes of Israel.

When the variety of quartz called amethyst is being formed, the water supplying the nutrients for the growing crystal carry some dissolved iron in its ferric (Fe+3) state. This is incorporated into the structure of the growing crystal. When the ferric iron is incorporated into the structure of the quartz crystal it forms “color centers”. Natural ionizing radiation in the ground then strips an electron from the ferric iron atom; this activates the color center giving amethyst its color. That means that the color center absorbs all the colors except those in the purple/violet region which are then transmitted and reflected to our eyes. The intensity and color of amethyst depends on how much iron is in the quartz crystal, where it is located in the atomic structure of the quartz, and the possible presence of aluminum which may impart a smoky color to the amethyst. Well, that’s the general idea at any rate. Want to know more? No? Well, on the off chance that you do, refer to A. Cohen & F. Hassan’s excellent article in the 1974 Canadian Mineralogist, Vol.59, pp 719-728.

Where is amethyst found?
There are certainly well over a thousand localities in the world that have produced amethyst specimens of varying quality. There are many localities for amethyst in North America. Thunder Bay, Ontario, Canada is well known and in some circles even cherished for the modest specimens of small drusy amethyst crystals found there. To a mother, even ugly children are beautiful. In South Carolina near the town of Due West, some spectacular specimens of amethyst have been produced. To see pictures of amethyst from various localities in the United States, click on the following links and scroll through the various quartz localities. The links below will show you what good specimens of quartz from various localities in the United States look like and as you scroll through them you will see that some of them produce fair to quite good specimens of amethyst.

http://www.mindat.org/mesg-95-147881.html
http://www.mindat.org/mesg-95-131705.html
http://www.mindat.org/mesg-95-146899.html

Mexico is well known to collectors for two amethyst localities that produce beautiful prismatic crystals of amethyst. Most amethyst localities produce crystals that are not long and slender like these. The two localities are Mun. Las Vigas de Ramírez (Mun. de Profesor Rafael Ramírez), Veracruz and Amatitlán, Mun. de Zumpango del Rio, Guerrero. Both localities produce fine examples of prismatic amethyst crystals.


Amethyst from Las Vigas, Veracruz, Mexico

Amethyst, Amatitlan, Guerrero, Mexico


Only a few localities are rich enough to be mined for amethyst on a regular basis. Some of these are mined almost exclusively for gem material and at those mines good specimens are rarely preserved. Only a handful of localities are mined mostly for specimens, and the miners keep a sharp eye out for any material which can be cut into gems. Any beautiful amethyst specimen with a substantial amount of high-grade gem material seldom ends up in a mineral display, but will most likely be hammered down into gem rough and cut into anonymous faceted stones which will sell for more than the intact specimen would have brought.

Where is most of the world’s amethyst produced?


Amethyst geodes offered for sale by one of the large amethyst and agate wholesalers in Soledade, Rio Grande do Sul, Brazil



More amethyst geodes offered for sale. They have been cut in half and the sawed surfaces polished.


The most prolific locality in the world for amethyst specimens and agate nodules is found in Rio Grande do Sul (RGS), the southernmost state of Brazil. One could argue that this area has produced ten times more amethyst than any other amethyst locality and perhaps as much as all other amethyst localities in the world combined. Many places in Rio Grande do Sul produce amethyst and agates, but today the big producer is the region around the little town of Amethista do Sul (formerly Sao Gabriel), near the larger town of Irai. This region is not far from the Argentine border and about 250 miles NNW of Porto Alegre, the capital and largest city in Rio Grande do Sul.
This region produced amethyst and agate during the last part of the 19th century, all during the 20th century and is still going strong. The total production of the area can be measured in kilotons. Knowledgeable individuals place the current production of amethyst specimens at two to three thousand tons a year. Originally, the farmers in the region found the amethyst and agate loose in the soil of their fields and (especially agates) as cobbles and boulders in streams and rivers. Later the richer areas of alluvial material were mined for agate and amethyst specimens. Later yet, when the easily worked area began to play out, miners began to work the host rock and produce material (especially amethyst) from open cut mines by drilling and blasting the basalt. Today, most of the mining takes place underground, about which more shall be said.

The geology of the region is mostly sheet basalts weathered into a rolling countryside with occasional rivers and streams, now a rich, green farming area with corn and black beans predominating. Sheet basalts are rock formations that form when cracks in the earth open and molten rock pours out over the earth’s surface like a big thick blanket. Each flow can be a few feet to over a hundred feet thick and can cover many thousands of square miles. Some of these flows occur again and again, building up over time to depths of many thousands of feet. Basalt is an igneous rock and in Southern Brazil is black to gray on a fresh break. Some of the basalt flows in Rio Grande do Sul are are rich in gas and form gas bubbles that are trapped inside basalt flows when they cool and harden into rock. In Rio Grande do Sul the size of the bubbles (vesicles) ranges from microscopic to several feet in diameter (less than 3 meters) and as much as five or even six meters tall. These vesicles tend to be typically rounded, and taller than they are broad. They are somewhat flat on the bottom, and usually taper to a narrower rounded top, very much in shape like the cone heads of television sitcom fame. Sometimes these cones become distorted along one or more axes or run together and occasionally vesicles with two or more “cone heads” are found. If these can be taken out intact, which they usually are, they are cut in half along the long axis of the geode and form half "cone heads" which are commonly called amethyst cathedrals.

How are amethyst geodes formed?
Basalts are porous, and over time, water laden with dissolved silica and other minerals migrates through the basalt formations, fills the trapped bubbles (vesicles) and slowly forms minerals of various kinds on the rock walls of these ancient gas bubbles. In the basalts of Rio Grande do Sul, the mineral that most commonly lines the walls of these vesicles is the variety of quartz called agate. Then later, quartz crystals grow on the agate. These quartz crystals are irradiated with natural radiation in the ground which turns them into amethyst.
I know you have always wanted to know what an amygdaloid is, so now I am going to tell you just in case you want to show off a little: When the vesicles become mineralized they are called amygdaloids. In Rio Grande do Sul some amygdaloids become filled entirely with minerals, typically quartz or the variety of quartz (silica) called agate. Over millions of years the rocks containing these amygdaloids can weather away, and if the minerals filling the amygdaloids are more resistant to weathering than the rock, and quartz and agate certainly are, then these amygdaloids are left in the soil and on the surface of the ground. At this point they are called nodules. If the material in the amydaloid was agate they are called agate nodules. Simple, right? However if the agate fills only part of the amygdaloid with its center still empty it is called an agate geode. Geodes are commonly more or less round and when you break them open they are hollow and frequently lined with crystals, commonly quartz crystals. If the quartz is of the amethyst variety, they are called amethyst geodes. It is really rather simple minded. In Rio Grande do Sul, the miners are not willing to wait several million years for natural weathering processes to remove the rock from around the amethyst geodes, so they drill and blast the rock away as efficiently as possible. Of course, few of the amethyst geodes would ever survive millions of years of natural weathering.

How do they find these amethyst crystals?
Near the little town of Amethista do Sul, one particular basalt flow below the town has been found to be particularly rich in amethyst. This basalt flow, about 10 feet thick, is just one of many sandwiched between others and is not much different in color or texture from them. However, it has a lot more amethyst filled amygdaloids than the others. Like most of its cousins in the region, this basalt flow is relatively flat so, once it is located, miners can easily track its outcroppings on the hillsides or in the walls of valleys. It also means that when men drive tunnels into the basalt searching for amethyst, they can drive level tunnels level into the formation and not have mine up or down dip. This makes mining relatively easy. The tunnels are driven into the basalt parallel to each other, and some tunnels have been driven 800 feet into the basalt. Only natural ventilation removes the fumes from blasting and the exhaust from the little diesel powered jeep-like dump trucks needed to remove the waste rock from the tunnels. Ventilation is helped by the large number of little cross-cuts that interconnect the tunnels. In rich areas, all the rock is removed except for pillars left in place to keep the mines from caving in. Being underground in the mines is a little like being in a vast room with a low ceiling and arch pillars as far as you can see.


A typical amethyst mine. This one is in Urugyay near the little town of Artigas on the border with Brazil


Men using rock drills, powered by compressed air, drill holes into the basalt and blast it free sometimes using homemade black powder. Often while drilling, the drill will lurch forward into the rock indicating the drill has penetrated into the empty space in the center of a geode. The rod of drill steel with its carbide tip is withdrawn from the hole and a light is inserted. The light usually consists of a little flashlight bulb with wires soldered on and connected to a car battery. They use this light to try and get some idea of the shape of the amethyst geode and the quality of the amethyst. Knowing how big it is and what shape it has will make removal of the geode easier. If the amygdaloid contains only white quartz or very pale amethyst, a decision may be made to ignore it and just keep drilling and blasting rather than try to carefully remove it. The removal of amethyst geodes from the basalt, once they are drilled into, may involve a little more careful drilling and blasting but, more than anything else, it involves a lot of hand work. Pneumatic rock chippers and/or hammers and chisels are used to break away the surrounding basalt until at long last the geode is free. Sometimes the geode is broken or partially broken during the process. All geodes, to some extent have natural cracks which are made worse during removal from the rock. Also, they each have a drill hole in them which bears mute testimony to their method of discovery.
When the geodes are first removed from the rock, they look mostly like olive green coneheads ranging in size from a few inches to six feet high. They are green because they are covered with a thin green skin (1 or 2 mm thick) of a mineral called celadonite, which is a member of the mica group of minerals. This mineral is rather soft and flaky which makes it possible to easily break the basalt away from the body of each geode. The amethyst geodes that are recovered in a similar manner in northern Uruguay do not have this coating of celadonite and removing them intact from the rock is much harder. Even before the geode is completely removed from the basalt, some miners use various kinds of “super glue” to stabilize the geode. After the geode is removed from the mine, more glue may be used to help it survive cutting, polishing and truck transport. At the mines, most of the geodes are sold in large lots to one of the large companies dealing in amethyst. Most of these large companies are located in the city of Soledade, a few hours to the south by highway in Rio Grande do Sul. The geodes are commonly transported in open trucks full of wood chips from local saw mills. The wood chips make good packing material which is used again when the geodes are shipped out of Brazil to traditional markets in the USA, Europe and more recently Hong Kong, Taiwan and China and other places on the Pacific Rim.

The preparation of amethyst geodes for sale.
Most of the geodes are cut and polished in some fashion or at least prepared in some way for sale to customers. Most of the geodes are put in diamond saws and cut in half lengthwise to produce two more or less matching amethyst “cathedrals”. Others, especially those with very thick skins or rinds are cut perpendicularly to their long axis into amethyst “rings”. These diamond saws are of various sizes. The largest of them is a giant that can accommodate a blade almost 7 feet in diameter. The cut surfaces are then sanded down, polished (the large geodes are sanded by hand held sanding and polishing machines that look like the ones that are used to wax and polish cars), and sorted into various US$ per Kg categories. The prices increase with the intensity of the amethyst color. Small size geodes are always much more in demand so those in the 12 to 18 inch range also cost more per kg than larger ones, all other considerations being equal. Geodes with good bilateral symmetry cost more and those with less symmetry. Those with thick rinds also sell for less per Kg. Amethyst geodes that have very thick walls can weigh three times what other geodes of the same size but with a thin skins. A few geodes may have beautiful agate under their amethyst. Others may have striking inclusions of golden goethite needles in the amethyst or beautiful crystals of calcite or transparent gypsum crystals growing on the amethyst. All these factors will increase the value of the geode.

Most people want their amethyst geodes to:
1. Have dark colored, brilliant shiny amethyst.
2. Have thin walls so they won't weigh so much; remember, they are all sold by weight.
3. Have well formed amethyst crystals of good size.
4. Have good symmetry or regular form. That is, if you draw a line vertically up the middle of the amethyst cathedral, you want the left side to be a mirror image of the right side.
5. Be not too large. Large ones weigh a lot and therefore cost more. There is a much larger market for smaller ones. Unfortunately, nice small amethyst geodes less than 12 or 18 inches are not found in large quantities.
6. Be free of cracks, drill holes and other defects.

The grim reality and sales gimmicks.
Less than one geode in a hundred, perhaps one in a thousand is free of defects. In practical terms you must assume that all geodes have been repaired or reinforced in some way. The companies that sell the geodes have developed methods of fixing up geodes by one means or another to make them more salable. They fix up the drill hole by laying the amethyst geode on a big plastic sack filled with tiny tumbled stones, with the drill hole down. They then fill the hole with small crushed quartz fragments to the level of the base of the amethyst crystals. They then take loose amethyst crystals that match the size and color of the ones in the geode and seat them in the crushed quartz and arrange them till they have a good fit. Finally, they let epoxy resin run down over the loose amethyst crystals and saturate the crushed quartz in the hole below and let it harden to lock the repair in place. Cracks reinforced with glue are easily spotted from the outside because the glue turns the earthy olive green celadonite to almost a black color. Amethyst suppliers in Rio Grande do Sul remove this unattractive feature by painting the outside of the geodes with a thin green plaster that mimics the natural green celadonite coating in color. This effectively covers up any filled drill holes and all the reinforced cracks. If you look carefully at the cut and polished front of a geode you may be able to spot some of the more obvious cracks in the geode. These cracks occur naturally in almost every amethyst geode and may not correspond to any crack in the geode that has been reinforced with glue. It is difficult to spot the glue inside the geode.
Another trick used to sell amethyst is not to show it in sunlight or under a strong tungsten filament light. In this kind of lighting the amethyst will look a whole shade lighter in color. Indirect low-level sunlight seems to be the best environment to give the amethyst its darkest color. If the amethyst is sold in direct sunlight it will look much lighter than the same material sold in deep shade.

So is that all the bad news about amethyst and amethyst geodes?
No, I am sorry to say, it isn’t. The color in the amethyst from Rio Grande do Sul is elusive. That means that it will gradually fade in color. You definitely do not want to put amethyst in direct sunlight for any length of time. In about six months of exposure to sunlight, it will fade to a very pale amethyst or a gray looking quartz. This pretty much eliminates this material for use in outdoor gardening and architectural applications. Displayed in indirect low-level light the color will last a lifetime.
There have been no thorough scientific studies done on the fading of amethyst but I believe that amethyst from all localities fades on long exposure to light. All old jewelers have had the experience of having a customer come in and show them an amethyst ring and say something like, “This ring belonged to my grandmother, but when I was young I remember it as being darker.” Almost certainly it was. In the Winter Palace museum in St. Petersburg, Russia there is on display a number of antique gold and jewel encrusted bibles and icons. Some of the stones in the gold settings are almost gray looking. A hundred years ago they were vibrant, deep colored amethysts for which Siberia is famous.

Is there any good news?
The good news is that amethyst is a beautiful color, and the amethyst from Rio Grande do Sul, Brazil is the most “bang for your buck” you are going to find in the mineral kingdom. Everything else is going to be less attractive, more expensive and/or harder to get. In addition, many of its competitors in the mineral kingdom have their own problems, if only the person you buy them from would tell you.

Amethyst specimens are for the most part real tough. If you drop them on the concrete from the height of a table you are likely to end up with smaller pieces, but even these smaller pieces have value and can be sold for something. Try the same thing with a fancy piece of porcelain! When you need to clean your amethyst geodes you don’t have to worry about damaging them. You can take them outside, hose them off, scrub them with scouring power and your choice of brushes. You can even use metal wire brushes if you wish since the amethyst crystals are so hard and strong you won’t hurt them. However you might rub off some of the metal onto the amethyst crystals and this might change the color of the crystals a little. If you know how to use hydrochloric or nitric acid safely, you can use it to remove the metal without hurting the amethyst. You would be advised to carefully neutralize any acid which remains on the specimen. Residual acid won't hurt the amethyst, but it might eat up your carpet or whatever else it might be sitting on or is close to it. You certainly do not have to worry about the long term effects of air pollution degrading the specimen like you might for some fabrics, paintings or buildings built from sandstone or marble.

For many years the golden needles that were found associated with some of the amethyst crystals from Rio Grande do Sul were thought to be cacoxinite, but recently it has been demonstrated that some and most likely all of them are tiny prismatic crystals of goethite.

Some years ago this article was prepared for the customers of Jewel Tunnel Imports so they could know something about the amethyst they were buying from Brazil and/or Uruguay. It was aimed at that audience and not the more sophisticated audience that finds its way to mindat. As time permits, Ill modify and upgrade this article and put more pictures in it.
[Rock Currier 2013]
[Reviewed and proof read by George Holloway 2013]




Article has been viewed at least 66152 times.

Comments

Thanks for taking the time to prepared this article Rock! This was an excellant article and very interesting.
Bob

Robert Simonoff
3rd Mar 2010 2:30pm

In order to leave comments to this article, you must be registered
Mineral and/or Locality  
Search Google  
Copyright © Jolyon Ralph and Ida Chau 1993-2014. Site Map. Locality, mineral & photograph data are the copyright of the individuals who submitted them. Site hosted & developed by Jolyon Ralph. Mindat.org is an online information resource dedicated to providing free mineralogical information to all. Mindat relies on the contributions of thousands of members and supporters. Mindat does not offer minerals for sale. If you would like to add information to improve the quality of our database, then click here to register.
Current server date and time: December 19, 2014 01:35:02
Mineral and Locality Search
Mineral:
and/or Locality:
Options
Fade toolbar when not in focusFix toolbar to bottom of page
Hide Social Media Links
Slideshow frame delay seconds