Agate, a member of the quartz family formed in the recesses of volcanic rocks, is found in a variety of colors: red, green, gray, brown and striped. Since antiquity, agate has been one of the most popular stones for talismans and amulets, and remarkable powers are ascribed to each of the different varieties. The use of agate amulets has continued unabated into modern times, though early examples exist from the Sumerian and Egyptian cultures dating back four millennia before Christ. In addition to jewelry, agate was carved into cups, dishes and works of art. The Roman naturalist Pliny records that pharmacists favored mortar and pestles crafted from agate, a practice that is still common. By far the most preferable of the agate gems is the brown, or "tawny" type. It is believed to cure an enormous number of ailments, including fevers, epilepsy, schizophrenia, intestinal troubles, rheumy eyes and difficult menstruation. It is a good antidote for poison, and is said to quicken the intelligence. Green agate is excellent for quelling eye diseases and preventing sterility. Grey or white prevents neck problems and stomach upset; while red (or blood) agate was used by the ancients as an antidote to poisonous insect bites and other ailments spread through the circulatory system. All varieties of agate are useful for curing insomnia and inducing pleasant dreams, and are thought to have a universally positive effect on those who wear them.