This fossilized substance (used as a dietary supplement) has been found to be a plentiful source of easily-absorbed calcium, magnesium and other acid-buffering minerals. In fact, this fossilized substance was one of the first "antacids“ used by ancient man to relieve heartburn. This fossilized substance contains nearly 70 different minerals and trace minerals, with calcium found in the highest concentration and in the proper 2 to 1 ratio with magnesium. What is this dietary supplement?