Missy Fleming, age 14, of Birmingham, Ala., for her question:
WHERE DO WE FIND OPALS?
Opal is a gem stone that contains a rainbow of colors. But it is made mostly of the elements of common sand, or silica. The most prized opals are the deep glowing black gems from New South Wales, Australia.
The finest of the black gems glow with brilliant flashes of reds and yellows in addition to greens and blues.
Precious white opals are found in Europe, Queensland and Mexico.
An opal's background color may be black, brown or white. But cut and polished opals reflect many colors when they are held to the light.
The opal is unique among gems stones because it is not found in nature in the form of crystals. Instead, it is found in irregular patches, often filling cavities in rocks. Geologists speak of an opal as a hydrated silica gel, because it contains water along with the silica. The water content makes some opals a risky buy. Many of them check or crack after a long time in dry air. The checking occurs as the water in the gel dries out.
Most scientists believe the color flashes are caused by the water in the gems. Each layer of silica gel in an opal has a different index of refraction. That is, it bends light at a different angle. The angle varies according to the amount of water in the layer.
Scientists believe that these different bendings break up the light that strikes the stone into its rainbow colors.
There are many different kinds of opals. They are classified according to the color of their background and the brilliance of the light rays that they reflect.
Opals that give off brilliant flashes of color are precious opals. The black opals of New South Wales and the white opals of Europe and Mexico are examples of precious opals. The common opal is usually not a precious stone and shows no colors. The girasol is a precious opal. Its background is bluish white with reddish reflections. The fire opal is a variety of common opal. It is red, often without color flashes.
Other common opals may be transparent, and red, brown, green or yellow. Some opals are almost colorless.
Since the beauty of the opal lies in its internal color flashes, it is never cut with facets, like a diamond. Instead, it is cut with a gently rounded convex surface.
Large opals of the world include a Hungarian opal that weighs 594 grams and is now in the Museum of Natural History in Vienna.
The Roebling opal, which was found in Nevada, is the most beautiful American opal. It is almost pitch black with color flashes of great brillance. It weighs 530 grams and is on display at the National Museum of Natural History in Washington, D.C.
The huge Desert Flame of Andamooka opal weighed over 6,800 grams when found in Australia in 1969. It was split in two.