Amethyst is the violet or purple variety of Quartz. The Amethyst geodes from southern Brazil are found in volcanic rock or basalt. When forming, gas bubbles rise towards the surface and when the lava cools quickly, it thickens and prevents the bubbles from reaching the surface. Cavities are formed as the lava hardens around the bubbles. Over many of thousands of years silica rich liquid with trace amounts of Iron found its way into these cavities and Amethyst geodes were formed.