Opals are the birthstone for October.
Opals are famous for their “play of colors” – many stones flash the colors of the rainbow when moved, due to the interference of light on small cracks and other internal structural differences. Opals also have characteristic colors due to impurities within the stone. The milky or pearly appearance of some opals are due to inclusions of tiny gas bubbles. Yellows and reds betray the presence of iron oxides. Fire opals were first mined in Mexico, and continue to be produced today. In the United States, brilliant fire opals are also found in Nevada.