Susan Dee Thomas, age 13, of High Point, North Carolina, for her question:
Is it true that opals come in different colors?
The ones we see most often are the so called common opals that gleam like milky moonbeams. But there are rarer kinds that display all the tones and tints of the rain¬bow. The rarest and most precious is the black opal that shimmers like the watery mirror in a deep dark well. The various opals are classed as precious or semiprecious gem stone. The most valuable ones are Judged on durability, plus their embedded specks of flashing color.
The basic material in opals is silica, a compound of oxygen and silicon. This same abundant mineral is used to make sand, various forms of plain and crystal quartz and a rainbow assortment of agates and other semiprecious gem stones.
The silica used to create opals is mixed with minuscule molecules of moisture. This mineral is not as hard as the solid silicates and only about twice as dense as water. The two basic ingredients are deposited by warm seeping water, rich with dis¬solved chemicals. Together they form a glassy stone which may be transparent, trans¬lucent ox opaque. The background color may be white or milky blue, brown or golden yellow, gleaming grey or midnight black. Even when the background color is quite plain, a polished opal has a soft, elegant luster. The most valuable plain color is the pre¬cious white opal, with a serene milky shine on its face.
The plain colors are lovely until you see the opals that have embedded specks of color. When you gaze deep into the glassy stone, you may see flickering flecks of pastel pinks and blues. Other types wink back with flashes of orange and tiny flames of fiery red. The fabulous fire opal has a basic color that ranges from hyacinth red to honey yellow. It may be plain or embedded with a variety of flashes that look like fairy campfires.
The common opal is plentiful in petrified wood and around hot springs. Fire opals and other precious beauties are found in Mexico and Australia and most of the precious whites are found in Hungary. The glamorous black opals are found in New South Wales and in Virgin Valley, Nevada.
The winking colors, we are told, are caused when embedded moisture refracts the light. Different wave lengths of light are bent at different angles and separated to reveal glimpses of their rainbow colors.
Opal jewels are not faceted like diamonds to reveal an array of glittering faces. Usually, they are shaped into domes or oval cabochons to show smooth curved surfaces Their embedded colors are flashiest when you gaze down through the hump or hold the glassy stones up to the light. The common and semiprecious types are lovely and not too expensive. But the price of a rare precious opal is very high.