The name is a derivitave of the Indian word "stone". There are three groups of Opal:
the opalescent precious Opals, special for their rainbow iridescence which changes
with the angle of observation; the yellow fire opals, named after their orange color;
and the common opal, which is mostly opaque and without play of color. It is most common.
All opals have a measure of water in them ranging up to 30%. Opals can lose their
water, thus dulling their color. The water can escape through cracks in the stone or
evaporate from even a little heat. Their color can be temporarily restored by saturating
them with water or oil. Opals are very brittle and are sensitive to knocks, pressure, heat
acids, and alkalies.
Black: usually none
Fire: greenish to brown