While feldspar is one of the most abundant mineral families in the world, gem quality crystals are scarce, coveted and spectacularly beautiful, as they often possess rare optical effects. Many feldspar gems only occur in isolated deposits and are far rarer than better known gems such as diamond, ruby or sapphire. Some varieties are commonly encountered and many are seen in granites, for example, “Black Pearl” granite countertops are actually composed mostly of feldspar. While orthoclase, a potassium-rich feldspar, is better known to collectors, there is one variety that many are familiar with—moonstone, prized for its shimmering adularescence.