Agates are composed of a cryptocrystalline silica (a fine form of chalcedony). They form in veins or nodules in secondary container rocks. Agates ares found in a range of natural colors and take a good polish. Frequently, banding or other patterns show well when the stone is cut and polished.
There are many trade names under which agate is sold, depending mainly on its appearance and sometimes the locality from which it comes. There is no scientific rationale to these many trade names.
Agate has a global distribution. Agate is not synthesized, but it is frequently dyed to improve or change its color. Here's an excellent blog post entitled The GemHunter's Guide to Agate and Jasper.
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