Often called synthetic stones or simply synthetic, the latter is man-made and is not extracted from the earth. Generally speaking, the physical and chemical properties coincide approximately with those of their natural counterpart. Not being from the earth, a synthetic will always be less expensive than a natural gem. There is virtually no gem today that cannot be synthesized for commercial sale and more than a dozen synthesis processes are known worldwide. Emeralds, rubies and sapphires are highly prized in synthesis because of the high price (rarity) of their natural counterparts.