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. What is lead crystal? Lead crystal is the term used for glass that combines silica, potash, cullet AND lead oxide. The lead content softens the glass and makes it easier to cut. At the same time, the product becomes more sensitive to light and acquires its characteristic and inimitable sparkle.