68 A Book of Precious Stones
	besides its characteristic of an uneven and ir­regular distribution of colour, it is unique geo­logically, for it occurs exclusively in its primary situation, that is, in the rock in which it was formed. It is one of the minerals characteris­tic of crystalline schists, and is frequently found embedded in mica schists and similar rocks. The magnificent beryls found at Muzo, Colombia, however, are an exception; there the emeralds are embedded in calcite veins in lime­stone. Emeralds are never found in gem gravels, like diamonds, rubies, sapphires, and other precious stones. The ancient source of the emerald was Ethi­opia, but the locality is unknown. From upper Egypt, near the coast of the Red Sea and south of Kosseir, came the first emeralds of historic commerce. There is a supposition that the emerald beryl was first introduced commercially into Europe just prior to the seventeenth cen­tury from South America. Emeralds had been found before this, however, in the wrappings of Egyptian mummies and in the ruins of Pompeii and Herculaneum. Ancient Egyptian emerald mines on the west coast of the Red Sea were rediscovered about 1820 by a French explorer, Cailliaud, on an expedition organised by