Polished gemstones are sold in 1/4 pound lots (approx 40 to 60 pieces) and are natural, undyed material that has been tumble polished for use in mosaic art. The piece size ranges from approx. 1/2 to 3/4 inch. Gemstones have natural variation in their exact color, pattern, and shape. The process of cutting and polishing gems is called gem cutting or lapidary. Gemstone material that has not been extensively cut and polished is referred to generally as rough. Rough material that has been lightly hammered to knock off brittle, fractured material is said to have been cobbed. All gems are cut and polished by progressive abrasion using finer and finer grits of harder substances.
Diamond, the hardest naturally occurring substance, has a Mohs hardness of 10 and is used as an abrasive to cut and polish a wide variety of materials, including diamond itself. Silicon carbide, a manmade compound of silicon and carbon with a Mohs hardness of 9.5, is also widely used for cutting softer gemstones. Other compounds, such as cerium oxide, tin oxide, chromium oxide, and aluminium oxide, are frequently used in polishing gemstones. After a gemstone is sawed and ground to the desired shape and sanded to remove rough marks left by coarser grits, it is usually polished to a mirror-like finish to aid light reflection from the surface of the stone.