Dolomite

Dolomite is the name of a sedimentary carbonate rock and a mineral, both composed of calcium magnesium carbonate CaMg(CO3)2 
As a mineral, it is found in crystals. Dolomite rock (or 'dolostone') is mostly made of the mineral dolomite. 
Limestone that is partially replaced by dolomite is referred to as "dolomitic limestone". In the old U.S. geologic literature it is called "magnesian limestone". Dolomite was first described in 1791 for exposures in the Dolomite Alps of northern Italy.