|South America||Irai, Rio Grande do Sul, Brazil||9" x 9" x 3 ½"|
There are five amethyst specimens in this display, which might lead you to believe that amethyst is fairly common. You'd be correct. The mineral occurs in particularly showy pieces in many localities. This particular piece is different from the earlier amethyst specimens in the display - it is not from an essentially granitic rock.
Rio Grande do Sul, in Brazil, is home to the Parana basalts. Basalt is the black, volcanic rock seen in abundance in the western states of the U.S. The Parana basalts were erupted rapidly about 135 million years ago as a type of eruption called a continental flood basalt, or CFB. A CFB is a thick layer of basalt that has flowed over a large area, perhaps in multiple episodes. As the Parana flows depressurized and cooled, bubbles formed from dissolved gases, qualitatively like gas bubbles forming when a can of soda pop is opened. Some of the bubbles grew to impressive size, some over a meter in height and elongated, and amethyst was deposited on the bubble walls from the magmatic gases themselves. Although crystallization models of similar features elsewhere infer a role for local groundwater, the geodes in the Parana basalts may have formed entirely from magmatic gases.
We just used the term "geode." What is a geode? A geode is a crystal-lined cavity in rock, often but not necessarily sub-spherical. The crystals are mostly some variety of quartz or calcite, but geodes containing other minerals, or more than one mineral, are known. "Geode beds" occur in ancient lake sediments south of Dugway Proving Ground, Juab County, Utah. They originally formed in volcanic rocks and were re-deposited by erosion. Of course, you don't know what is inside them (if anything) until you break them open. Geodes can be purchased in most rock shops, and often they have very unusual colors for the cavity-lining mineral. These are the ones that have been artificially stained. If you want a "pretty rock," buy one. If you want an authentic mineral specimen. Steer clear of the bizarre colors.