Dolomite. Dolomite, calcium magnesium carbonate, is a common mineral that occurs throughout the world. Thick beds of dolomite were produced by the alteration of limestones (made of calcite, calcium carbonate) by magnesium-rich hot solutions, a process called 'dolomitization'. Dolomite forms good crystals in veins in ore deposits. Crystals also occur in open spaces within massive dolomite beds, such as the specimens recovered from the Simplon Tunnel in Switzerland. Both dolomite and calcite typically form crystals with a characteristic rhombohedral shape.