What is crystal? Natural mountain crystal, which is found in the hollows of rock formations, is a semi-precious stone of great hardness. A very poor heat conductor, it"s constantly cool to the touch. Because of this property, and the stone"s visual resemblance to ice, it was called "crystal" after a Greek word meaning ice. The crystal we find in the gift and tableware market is glassware, called crystal because of its clarity, fine quality and high refractive power. Lead oxide is often added to the original ingredients of silica sand, potash and cullet - although some products, justifiably called crystal for their purity and beauty, do not necessarily have to contain lead oxide.