Agate is the name given to a wide selection of varieties of chalcedony, a form of quartz which
is cryptocrystalline. Microscopically small crystals of quartz are embedded in opal, which is
itself hydrated quartz. The proportion of quartz to opal varies, and this variation is
reflected in differing densities, refractive indexes and other properties. The quartz crystals
are in a fibrous form, sometimes running approximately parallel to each other, sometimes
entwined.
The names for chalcedony and agate are somewhat interchangeable, although the term agate is
usually used to describe the banded varieties of chalcedony. Many agates are naturally colored,
others are stained for decorative effect.
Varieties and Colors
Agate can be many different colors, and because its banding is so variable, different types
of it have been given many different descriptive names. It is often used to make quite large
decorative items. Some names include moss agate and picture agate. Many types of organic
material have been fossilized by agate, where the organic material has been replaced slowly
over a long period of time, so that the original structure of the original item is retained.
Fossilized or petrified wood is an attractive form of agate.
Geodes and Thunder Eggs
Agate is frequently found in the form of rounded nodules or geodes. When split or sliced,
these show their banding to good, sometimes startling effect. In some geodes there is a
central cavity, in others the center is hollow. This provides an important clue to the
formation of such geodes.
American Indians of the western United States called agate nodules "thunder eggs", an
interesting name for one of natures most delicately beautiful and unique gemstones.
