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. Moonstones are the most highly-prized of the feldspar gems. Their beautiful blue iridescence is known as adularescence. Flawless, clear, or translucent gems exhibiting a rich blue sheen are most valuable. Moonstones are typically cut as cabochons or carved to best show off this effect. The finest moonstone gems come from Sri Lanka, Burma, Tanzania and India. This 107.84ct moonstone from Tanzania is very large for a fine moonstone with such pronounced adularescence. The Tanzanian mine was located on the road to Gongoni near Kilosa in 2000. The deposit produced gem quality material from 2003-2006. This moonstone was donated in 2012 by Tricia and Michael Berns and is a major upgrade for the National Gem Collection.
