Magnesium is often replaced by iron and manganese in the composition of magnesite, but the iron content of natural magnesite is generally not high. Magnesite crystals belong to trigonal carbonate minerals. They are usually crystalline or cryptocrystalline, dense and massive. The latter is also called porcelain magnesite, white or gray-white, with yellow to brown iron content and glassy luster. It has complete rhombohedral cleavage, while porcelain magnesite has shell-like fracture. Moss hardness is 3.5-4.5 and specific gravity is 2.9-3.1.