A fine-grained variegated chalcedony (a member of the quartz family, named in 1570), usually with colours arranged in stripes, blended in clouds or showing moss-like forms. Agates are generally formed by water seeping into cavities in volcanic rocks and depositing mineral salts in layers which over centuries have hardened into varied-coloured bands of chalcedony. These bands are often concentric, producing beautiful patterns. Named after the River Achetes in Sicily, agate is said to have been the eighth stone in the breastplate of the ancient high priest. In historic times, water in which it had been placed was used for cooking and drinking in order to alleviate sickness. Thought to protect the wearer from danger, encourage boldness and stimulate success, it has been used as a cure for insomnia, to ensure pleasant dreams and relieve stress. It is thought to be of assistance in childbirth as well as in the treatment of disease. It may alleviate eyesight, hearing and hair problems and assist in cases of poisoning. Agate is the zodiac gem for Gemini (21 May to 21 June).