In this example dolomite replaced an existing rock, a limestone (therefore made of calcite, calcium carbonate). What remains of the limestone can be seen in the dark areas  surrounding the dolomite crystals. In this case the substitution most probably took place because magnesium-rich waters flowed over a calcitic sediment in the subsoil for a very long time, slowly dissolving the calcite and depositing dolomite, which is less soluble, in its place.