Schiller or Adularescence is what gives Moonstone its 'glow' or 'sheen', which is usually silver or blue.
Moonstone comes in several colors, the more common being white, gray, and peach. White Moonstone, with an especially strong bluish sheen, is often called Rainbow Moonstone, and is actually not a moonstone but another feldspar mineral called Labradorite; it is very popular to use in jewelry. The most valuable is the blue Moonstones from Sri Lanka, particularly as there are fewer being found thus driving up prices for this most beautiful of stones.
If there's a stone you would like to know more about - drop me a line! Have a mystery stone? Feel free to post a comment about it, including a link to a picture, and it could be featured on this blog!
Chemical composition: KAlSi3O8
Crystal System: Monoclinic
Color: Most commonly white, peach, or gray; also blue, pink, green, and brown
Habit: Prismatic, Tabular
Fracture: Irregular/Uneven, Conchoidal
Specific Gravity: ~2.55
A Guide to Rocks and Fossils by B. Busbey III, R. R. Coenraads, P. Willis, and D. Roots. Published 2002 by Fog City Press. ISBN: 1877019518
Gemstone.org – Moonstone. http://www.gemstone.org/gem-by-gem/english/moonstone.html
Mindat – Orthoclase. http://www.mindat.org/min-3026.html
Wikipedia – Moonstone. http://en.wikipedia.org/wiki/Moonstone_%28Gemstones%29