Chalcedony

Chalcedony is silica whose crystals are too small to see without magnification: a compact mass made mostly of quartz with a little intergrown moganite, which Wikipedia classes as a variety of quartz. It has a waxy lustre and lets some light through. It comes in many colours, but the commonest run from white and grey through greyish blue to browns that can be almost black, and stones on sale are often dyed or heated to deepen their colour. Agate, carnelian, chrysoprase and bloodstone are among its many named varieties. The name probably comes, through Latin, from Chalcedon, an ancient Greek town in Asia Minor, though that link is disputed. People were working it into tools in central Australia about 32,000 years ago, and Minoan seals cut from it at Knossos date to around 1800 BC. Cunningham assigns chalcedony to the Moon and to water. He calls it a protective stone for travellers and against psychic attack, worn to bed to keep nightmares and fear of the dark away, and used for peace, strength and success. Hall calls it a nurturing stone of goodwill that brings mind, body and spirit into harmony and turns melancholy into joy, and she associates it with cleansing and healing; these are her claims, not established medicine.