Geodes and Amethyst Caves
Nodules are roughly sphere-shaped rocks. Those which are at least partly
hollow are called geodes. The
cavities are formed when volcanic rocks have large steam bubbles inside them.
Over time, crystals may form lining the inside walls of the cavities.
The most common material found in the cavities of geodes is quartz. The
cavity in most geodes is surrounded by a layer of agate in colored bands which
gives a spectacular effect when polished.
If the quartz is deposited slowly with relatively few impurities, it will
line the cavity with sparkling crystals. If the quartz is deposited rapidly, the
cavity will be filled with varicolored bands of agate, which is an extremely
fine grained form of quartz.
The crystals which form in geodes may be any color. The most common are
white, smoky, gray, lavender and brown. Among the most spectacular are those with
purple amethyst crystals.
Geodes occur in relatively few places in the world but, in these places, they
are abundant. They are found in the Mississippi and Ohio river valleys in the
USA and the
Chihuahua area of Mexico. The main source for amethyst geodes is the Rio Grande
du Sol region in Brazil and the adjoining part of Uruguay. Huge geodes,
described as amethyst caves and cathedrals, have bee found in Brazil. The
largest are up to 10 meters tall.