The minerals in the feldspar family make up more than half of the Earth's rocky crust. Occasionally these common minerals form crystals that shimmer like the light of the moon or a rainbow on a soap bubble. Called iridescence, this phenomenon is caused by light scattering, or diffracting, off closely spaced layers in the feldspar crystals. The gems cut from these iridescent crystals are called moonstone, sunstone, and labradorite. This delicately carved 50.77 carat moonstone is from Sri Lanka. Moonstones are prized for their beautiful blue iridescence.