The Opal ranges from clear through white, gray, red, yellow, green, shore, blue, magenta, brown, and black. Of these hues, red and black are the most rare and dear, whereas white and green are the most common. The word opal comes from the Latin opalus, by Greek òpalliòs, and is from the same root as Sanskrit upálá[s] for "stone", originally a millstone with upárá[s] for slab. Opals are also Australia's national gemstone.

Reference: en.wikipedia.org