The composition of agate varies greatly, but silica is always predominant, usually with alumina and iron oxide. Chalcedony, carnelian, amethyst, common quartz, jasper, opal, and flint occur as layers in agate, and are spoken of as kinds of agate. There are also clouded agates, star agates, moss agates, etc., so called from their appearance. Agate can be finely polished and is much used for ornamental purposes. Hardness c.6.5, specific gravity c.2.6.
Agate was believed in various cultures to have certain beneficial properties. Jews used agate to avoid falling and Arabs to improve the blood. In medieval times it was an anti-snake medicine. North Americans Indians used it to make implements of various kinds. It is still a symbol of good health and long life.
Related category• GEOLOGY AND PLANETARY SCIENCE
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