Dolomite

Dolomite is a close cousin of limestone. It is a sedimentary carbonate rock or calcium magnesium carbonate which differs from limestone, or calcium carbonate, because of its magnesium content. 

Dolomite is just a bit harder than calcite making it a good pendant stone but not so much ring or bracelet. 

Dolomite used to be a confusing term. It could mean a mineral or thousands of feet of the rock a mountain range is made from. Now if a rock is more than 50% dolomite it is known as a "Dolostone".