Dolomite is a type of milled limestone that is a useful source of both calcium and magnesium on ceramic glazes.
Dolomite (aside from talc) is the main source of magnesium oxide in high temperature glazes and can be used as a high temperature flux and to promote crystal formations in glazes -- glazes that use this material often aim to achieve the often sought after "dolomite matte" surface. Also referred to as calcium magnesium carbonate or limestone.