Agate

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 coloured, others are stained for decorative effect.
Varieties and ColoursAgate can be many different colours, 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 fossilised 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. Fossilised or petrified wood is an attractive form of agate.
Geodes and Thunder EggsAgate 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 other the centre is hollow. This provides an important clue to the formation of such geodes.The Indians (sorry, Native Americans) of the western American states called agate nodules "thunder eggs", a wonderfully romantic and descriptive name.