God for us to design products, we are the porters of nature!
The most prolific locality in the world for amethyst specimen and agate nodules is in Rio Grande do Sul, the southern most state of Brazil. One could argue that this locality 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, near the larger town of Irai. This region is near 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 and during the 20th century as well. 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 hills. 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 from open cut mines by drilling and blasting the basalt. Today, most of the mining takes place in underground mines, 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 times to depths of many thousands of feet. Basalt is an igneous rock and those in Southern Brazil are black to gray on a fresh break. Some molten basalts are rich in gas and when they cool and harden into rock.
The gas bubbles are trapped inside the rock like air bubbles in Jell-O. In Rio Grande do Sul the size of the bubbles (vesicles) range from microscopic to several feet in diameter(less than 10 feet) and as much as fifteen or perhaps twenty feet high. These vessicles tend to be typically rounded, and taller than they are broad, flat on the bottom, and tapering 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 sometimes vesicles with two or more “cone heads” are found.
How are amethyst geodes formed?
Basalts are porous, and over time, water laden with dissolved silica and other minerals migrate through the basalt formations, fill the vesicles and slowly form 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 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. Some amygdaloids become filled entirely with minerals, typically the variety of quartz (silica) called agate. Over millions of years the rocks containing these amygdaloids can weather away, and the if the minerals filling the amygdaloids are more resistant to weathering than the rock, 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 amygdaloid 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 in the middle and frequently lined with crystals, commonly quartz crystals. If the quartz is of the amethyst variety they are called amethyst geodes. It really is really rather simple minded. In Rio Grande do Sul state, 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 except that it has a lot more amethyst filed amygdaloids than the others. Like all of its cousins, this basalt flow is relatively flat laying, 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 the amethyst geodes, they can drive the tunnels level into the formation and not have to up or down dip. The tunnels are driven in to the basalt parallel to each other, and some tunnels have been driven 800 feet into the basalt. Only naturally ventilation removes the fumes from blasting and the exhaust from the little diesel powered jeep-like dump trucks needed to remove the waste rock form 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.
Men using rock drills powered by compressed air, drill holes into the basalt and blast it free using homemade black powder. Often while drilling, the drill will lurch forward into the rock indicating the drill has penetrated the wall of an amygdaloid into the empty space in the center of the amethyst 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 to try and carefully remove it intact from the basalt. 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 chiseled 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 its method of discovery.
When the geodes are first removed from the rock they mostly look like olive green coneheads raging 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) of a mineral called celadonite, which is a member of the mica group of minerals. This mineral is rather soft and flaky and makes it possible to easily break the basalt away from the body of geodes. 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 state. 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 to USA, Europe and more recently to Hong Kong, Taiwan and other places on the Pacific Rim.
The preparation of the 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 or 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 10 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 than larger ones so small geodes, those in the 12 to 18 inch range also cost more, all other considerations equal. Geodes with good bilateral symmetry cost more and those with very thick rinds sell for less per Kg. Amethyst geodes that have very thick walls can weigh three times what another would geodes of the same size but with a thin skin. A few geodes may have beautiful agate under their amethyst.1 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 what their amethyst geodes to:
1. Have dark colored, brilliantly shiny amethyst.
2. Have thin walls so they wont weigh so much; remember they are all sold by weight.
3. Have amethyst crystals of good size and be well formed.
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 side on the left to be a mirror image of the right side.
5. Be not be 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, in fact hardly at all at this locality.
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 are 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 get 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. Recently however the suppliers have taken to painting the outside of the geodes with a thin green plaster, effectively covering up any filled drill hole and all the reinforced cracks. At a glance it this treatment from the natural olive green celadonite coating. 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 or 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 exposed 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 through 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 think it was darker.” Almost certainly it was. In the Winter Palace museum in St. Petersburg, Russia there are 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 100 years ago they were vibrant deep colored amethysts for which Siberia is fameous.
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 mineal 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 form 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. So you can even use metal wire brushes if you wish, the amethyst crystals are so hard and strong you won’t hurt them but 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 neutralized any acid which remains on the specimen. Residual acid wont hurt the amethyst, but it might eat up your carpet or what ever else it might be sitting on or was 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.
1. 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 that could know something about the amethyst they were buying from Brazil Brazil and Uruguay. It was aimed at that audience and not the somewhat more sophisticated audience that finds its way to mindat. As time permits, Ill modify and upgrade it and put more pictures in it.
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