The minerals in the feldspar family make up more than half of the Earth's 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 moonstones, sunstones, and labradorite. The iridescent property of labradorite results from light diffracting off closely spaced layers of calcium- and sodium-rich feldspar and is called labradorescence. As the stone is turned, flashes of blue, green, yellow, and red are visible across properly oriented surfaces. The highest quality labradorite is generally cut as cabochons or used for carvings, such as the cameo seen here. The most important sources of labradorite are Labrador (for which the mineral is named), Finland, and Madagascar.