Lazurite is one of the feldspathoid minerals. This is a group of minerals that form instead of feldspar when there is either not enough silica or too much alkali (calcium, sodium, potassium) and aluminum to fit into feldspar's molecular structure. The sulfur atom in its formula is unusual. Its Mohs hardness is 5.5. Lazurite forms in metamorphosed limestones, which accounts for the presence of calcite. Afghanistan is the source of the finest specimens.