Ann Adkins, age 15, of Lowell, Mass., for her question:
WHERE DO WE FIND THE BEST OPALS?
An opal is a gem stone that holds a rainbow of colors. It is made mostly of the elements of common sand, or silica. The world's most prized opals are the deep glowing black gems that come from New South Wales in Australia.
The finest black opals from Australia are lightened by brilliant flashes of reds and yellows in addition to greens and blues.
An opal's background color may be black, brown or white. But cut and polished, opals reflect many colors when they are held up to the light.
Opals 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 known as precious opals. The black opals of New South Wales and the white opals of Europe, Mexico and the Queensland section of Australia are all precious opals.
The common opal is usually not a precious stone and shows no colors. The fire opal is a variety of the common opal. It is red and 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 include a Hungarian opal that weighs 594 grams. It 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 brilliance. It weighs 530 grams and is on display in the National Museum of Natural History in Washington, D.C.
Opals are found in many different parts of the world.
Opals are not found in nature in the form of crystals. Instead, they are found in irregular patches, often filling cavities in rocks.
Gemologists talk of an opal as a hydrated silica gel because it contains water along with its silica.
The water that is found in opals makes the purchase of some of the stones a risky buy. Many of them crack after a long time in dry air.
Scientists say that the color flashes in an opal are caused by the water in the stone. Each layer of silica gel in an opal has a different index of refraction. This means that the light bends at a different angle in each stone.
The angle varies according to the amount of water in the layer. Scientists say that these different bendings break up the light that strikes the stone into its rainbow colors.
The opal is the birthstone for the month of October.