Alice! Dear Just wondering, Dolomite, or calcium magnesium carbonate, is a common mineral extract that occurs as crystals in large rock beds of limestone. Calcium carbonate is a cheap calcium supplement with a high percentage of calcium by weight, but the body has difficulty in breaking it down for use.lice! Dear Just wondering, Dolomite, or calcium magnesium carbonate, is a common mineral extract that occurs as crystals in large rock beds of limestone. Calcium carbonate is a cheap calcium supplement with a high percentage of calcium by weight, but the body has difficulty in breaking it down for use.