A blue type of plagioclase feldspar which was discovered in Finland during World War II when an explosion uncovered a blue flash from some rocks. Officially, spectrolite only comes from near the village of Ylämaa, Finland, although some very similar stones have been found in Russia and Madagascar (minerologists consider these stones to officially be moonstone rather than spectrolite). The "inner glow" (compared in one of my gemstone books to "a searchlight as it pierces a deep blue evening sky") is called labradorescence and the stone is usually cut in a flattened dome shape to maximize its appearance.