Donna Palmer, age 12, of Rockford, Ill., for her question:
Where do they mine opals?
Some opals rate as precious gems, along with blood red rubies,, misty blue sapphires and glittering diamonds. Though gem opals are rare, there are many kinds of semiprecious opal and opal minerals in the rocks of the earths crust. Andy has a piece of petrified ginkgo wood which is opal mineral. One very common opal mineral works for us in the kitchen. Most cleansing powders contain diatomaceous earth which is made from the shells of tiny sea dwelling diatoms. And these microscopic shells are made from silica, the same mineral which makes the precious gem opal.
Silica is also the basic mineral in quartz, sand, agate, flint, rock crystal and a host of other common and semiprecious stones. In opal, the silica is combined with maybe somewhat more or somewhat less than ten per cent of water. The final mixture is a kind of hard jelly of silica and water. It may form as a paper thin lining in porous volcanic rock. It may occur in lumps or nodules. A precious opal is usually a nodule no bigger than a hazelnut. In certain regions, these valuable nodules are found buried in bed rock.
In the days of ancient Rome, all the precious opals were taken from the mines of Cserwenitsa in Hungary. Not until modern times were rich new opal fields found in Australia. The first Australian mine discovered was in White Cliff, New South Wales. A few years later another field was found nearby on the border of New South Wales and. Queensland. In the New World, precious opals are mined in several parts of Mexico and South America and at least one precious opal has been found in Nevada and Idaho.
Common opal is waxy white until we expose it to ultra violet rays. Then it glows with a green fluorescence and continues to glow for a while after the light is turned off. Precious opals, though made of the same basic minerals silica and water, come in a wide variety of colors from soft moonglow, through the bands of the rainbow to glowing coals and blazing flames. The colors are formed by spidery cracks and by impurities in the stones. The look of frozen fire within a beautiful opal is caused by the bending or refraction of light.
The precious harlequin opal is a frozen blaze of pale starlight. Pastel flames of pink and blue flash from a multitude of miniature tile surfaces within the stone. This opal is found in Hungary. Mexico and Honduras. Fire opal is a frozen furnace of ruby reds and honey yellows. Precious opals from Japan are often blue, like a sunlit morning. But the most vivid of all of these gem stones is the black opal. It is said to capture the sparkle of a dewdrop, the colors of the rainbow and set them in the blackness of midnight.