Organized, homogeneous minerals. Rock collectors love to find crystals.

How Amethyst Cathedrals are Formed

purple amethyst cathedral in a museum with other minerals

Amethyst cathedral at the Sutton Museum. Photo by Stephanie Reed

Article by Dr. Bill Cordua, University of Wisconsin-River Falls

Have you ever been to a show and seen enormous amethyst geodes or crystals 3-5 feet or more in height? The tubular geodes are lined with deep purple gemmy amethyst crystals. How do such wonders form?

These excellent geodes come from a region along the Brazil-Uruguay border. The genesis of deposits on the Brazil side of the border has recently been extensively researched by an international team of geochemists lead by H. Albert Gilg of Techniche University Munchen in Germany (Gilg, et. al., 2003). The geodes are mined from several lava flows belonging to the Parana Continental Flood Basalt Province. This was one of the largest outpourings of basalt lava known. An estimated 800,000 cubic kilometers of lava extruded over an 11 million year time span. For comparison, this would be enough to cover Minnesota with a pile of basalt lava over 2 miles high. The lava outburst occurred as part of the opening of the South Atlantic Ocean during Cretaceous time about 130 million years ago. Of all these flows, however, only a few are known to host amethyst cathedral geodes.

Gilg et al. proposed a 2-stage model for their formation. In the first stage the large hollows form. This was caused as volcanic gases were released from certain lavas as they cooled. Not every lava has enough dissolved gas to form such big openings. As gas bubbles emerged from the congealing lava (much as bubbles emerge when beer or soda pop is poured) they coalesced as they rose. The lava was cooling fast too, and soon became so thick and sticky that bubbles quite rising and were trapped. The bulbous to tubular shapes thus point towards the top of the flow, a fact easily seen when the geodes are in place in the mines. These cavities, though, were empty of crystals.

The second stage was the formation of the amethyst, plus celadonite, calcite and gypsum fillings. An important clue to this event is the presence of small gas and liquid bubbles (called fluid inclusions) trapped within these minerals. These are samples of the mineral-forming liquids caught as the crystals grew. Fluid inclusions are treasure troves of information when studied with sophisticated instruments. Analyses of the fluid inclusions in the amethyst, calcite and gypsum show them to be filled with slightly salty water. This water had a temperature of no more than 100 degrees C, and possible less than 50 degrees C, during mineral formation. These cannot be fluids related to the magma that formed the lavas.

What was the source of these fluids? An amazing story unfolds from the radiometric dating of the minerals. The basalts formed about 130 million years ago, but the green celadonite, which makes up the rinds of the geodes, formed about 70 million years ago. For 60 million years these enormous cavities sat empty of crystals. Trace element data from the fluid inclusions gives another important clue to the source of the mineral-forming fluid. Below the lavas is a large aquifer (the Botucatu aquifer) filled with ground water that closely resembles the fluid inclusion liquids. Uplift and tilting of the area about 70 million years ago would force water out of the aquifer into the porous areas of the overlying lava. In the lava flow these waters would have found volcanic glass. Glass breaks down over geologic time and makes silica and other chemicals available in a form that is readily soluble in water soaking through the rocks. The water carried these chemicals into the cavities, where the amethyst and other minerals grew due to cooling and pressure release.

The special combination of geologic circumstances, unfolding over millions of years, is not often duplicated. Understanding the process gives geologist tools to prospect more efficiently for these wonders.

Gilg, H. et. al, 2003, “Genesis of amethyst geodes in basaltic rocks of the Serra Geral Formation (Ametista do Sul, Rio Grande do Sul, Brazil): a fluid inclusion, REE, oxygen, carbon, and Sr isotope study on basalt, quartz and calcite” Mineralium Deposita vol. 38, p. 1009-1025.

The Glacial Drifter 08/2011, The Gemrock 06/2015

Geode Cake

a white layered cake with a blue candy geode decoration

Cake by Whisk Cake Company,

A talented baker made this geode layer cake with fondant for the layers and a rock candy geode surrounded by gold and silver leaf. I am very impressed with the banding in the geode. It looks very tasty. This cake was recently featured on Cake Wrecks in their Sunday Sweets section, which is a day of beautiful cakes to contrast with the awful cakes on other days. There are other geode and rock-related cakes including a malachite cake posted that day, so be sure to go to the Cake Wrecks post “Oh Em Geode” and see the rest.

World’s Largest Uncut Diamond Up for Auction

hand holding largest uncut diamond from Lucara

Photo by Donald Bowers/Getty Images for Sotheby’s

From the New York Times, June 27, 2016:

In the fall of 2015, a 1,109 carat white diamond was found in the Lucara mine in South Africa. The diamond is called the Lesedi La Rona, which means “Our Light” in Setswana. On Wednesday, June 29, 2016 it will go up for public auction at Sotheby’s in London. Usually, precious stones like these are sold secretly by sealed bids and kept anonymous, so this Sotheby’s auction is unusual. This is the first time that such a large diamond has been on sale publicly.

The Lesedi La Rona is a type IIA diamond, which is very rare and is the most chemically pure type of diamond. Other famous IIA diamonds include the Koh-i-Noor and the Cullinan, which is 3,106 carats and is the biggest diamond ever found. The Cullinan was cut into 9 different stones and the pieces are now owned by various monarchs. Sotheby’s says that “whoever buys the Lucara stone will pay Sotheby’s a 12 percent fee, known as the buyer’s premium, on the hammer price for anything over the first $3 million, and a higher percentage of the first $3 million.” Whoever buys the stone will then have to decide if they want to have it cut. What a hard decision! What would you do?

The whole article is here:

UPDATE: It turns out nobody bought the diamond because the bids failed to meet the minimum reserve price. The reserve price was secret (just like at eBay) but it must have been more than £45 million ($61 million) because that was the highest bid. Maybe it will go on sale again later. Source:

Selenite Stories

Gray selenite crystal and orange selenite crystal, both about the size of a ballpoint pen.

Photo by Stephanie Reed

Selenite is a type of gypsum that has a flat reflective surface, usually gray, clear, white, or amber. Red is an unusual color for selenite, but they do exist. It is very soft and can be scratched by a fingernail (2 on the Mohs scale). At our February meeting, two of the door prizes were these selenite crystals found by President David Reed. The amber one is from Lake Kanopolis in Kansas. The gray one is from Lake Wilson, which is also in Kansas. David went to Lake Wilson and put a small crystal in the mud. He returned 3 years later and found the large gray crystal.

One time David and Stephanie went to Kansas and found several selenite crystals somewhere near the dam at Lake Wilson. They were gray like this one pictured, but much smaller. On the April field trip to Marquette, some club members also found selenite crystals.

Pyrope Garnet

This is a special type of garnet called pyrope garnet. The name comes from the Greek pyro, meaning fire. Pyrope and other members of the aluminum part of the garnet group have a higher specific gravity and hardness, and are usually red. Calcium garnets like the previously mentioned andradite and uvarovite are the ones that are usually green and have a lower hardness and specific gravity.

Pyrope garnet is difficult to distinguish from almandine, but pyrope usually has fewer flaws and inclusions. However, garnet jewelry is usually almandine garnet because almandine is much more common and inexpensive.

If you would like some pyrope garnet it can be found nearby in Kansas, all around the Nemaha Uplift (or Nemaha Ridge), which is in the area between Salina and Manhattan, and extending south into Oklahoma. Basically, garnets are found anywhere near previous volcanic activity. The one pictured is from Apache County, Arizona. They are also found in Africa and other places. For lots and lots of information about this particular specimen, see its page on the RRUFF here.

Andradite Garnet

andradite garnet

Credit: Aaron Palke/Gemological Institute of America

Since it’s January, it’s a good time to read about this garnet originally posted by Chemistry in Pictures.

“This gemstone isn’t pure andradite garnet [Ca₃Fe₂(SiO₄)₃], but its flaws produce its mesmerizing colors. Some gemologists think that this rainbow explosion arises because the garnet’s different elements aren’t regularly spaced from the core of the gemstone to the outside. For example, in some regions, aluminum atoms might have worked their way into the structure and replaced the iron atoms. These irregularities create mismatched sheets of atoms that then bend and stretch. This makes the stone birefringent, meaning that light travels through it at two different speeds. Under cross-polarized lighting conditions, rays of light that enter get misaligned by the time they exit, so they then interfere with each other and highlight some colors in certain spots, producing the spectrum seen here. The black flecks are tiny pieces of magnetite that were enveloped by the crystal as it grew.”

Chemical Composition of Gemstones

Here’s a neat infographic from Compound Interest (one of my favorite websites) that describes 16 different gemstones and why they have different colors. It also includes their chemical formulas and hardness on the Mohs scale.

Many gemstones would be colorless or a different color if not for the presence of small amounts of transition metals such as chromium or titanium. For example, you can see that aquamarine and emerald both have the same chemical formula Be3Al2(SiO3)6, but emeralds are green because of chromium ions replacing some of the aluminum ions and aquamarines are blue because of iron 2+ or 3+ ions replacing some of the aluminum ions. Click through to read the whole article, because there are many other ways that gems and minerals get their colors!