Opal is a form of silica. Water makes up from 6% to 10% on average of its content, which makes it advisable to protect it from extreme dryness.
It appears in shades of white, black, red, orange, green, blue, yellow and a rainbow of other colors, of which the most common are white and green, with the rarest being red in black. The colors shimmer due to the refractive structure of the material. The term "opalescent" is erroneously used to describe this phenomenon. In fact, that term refers to the milky look of a form of opal-like substance known as "potch", often found near opal deposits.
Opal veins tend to yield material that is very thin. In order to use this material in jewelry, a special technique often is applied to create "doublets" and "triplets." The former is accomplished by backing the opal with a black material, emphasizing the way the colors play for the eye. The latter also backs a thin slice of natural opal with a dark material and adds a thin, clear dome of quartz or plastic atop. The dome tends to magnify the color display while protecting the opal beneath it, which is inherently fragile.
There are other forms of natural opal that are seldom used in jewelry. These include milk opal, fire opal, Peruvian blue opal and boulder opal. These material are occasionally used in carvings.
Opal mining is hot, dry, dusty and often frustrating work.
About 97% of jewelry-quality, natural opal comes from mines in Australia. I have visited the opal mines in Coober Pedy, South Australia, which is a major source for natural opal. The otherwise barren, outback landscape is studded with deep vertical shafts and mounds of earth that has been extracted during the mining process. These earthen mounds are often picked through by visitors looking for potch that, while not usable for quality opal, may have some scraps of color. These make interesting souvenirs. We have a few pieces in our collection.
As an aside, "fossickers" - that's what they are called - are not advised to do so at night.
The holes often are unseen in the dark and they are very deep. Bye-bye.
An interesting aspect of Coober Pedy is the homes and rooms carved into the escarpment excavations left behind by mining. One is even a chapel. Because they are underground, they maintain a constant cool temperature, even at mid-day when the outside temperatures exceed 100 degrees.
Andamooka, also in South Australia, is a major source of black opal, as is Lightning Ridge in New South Wales.
Black opal is highly valued for the intensity of the color found in its black specimens.
Given the high cost of natural opal, most opal in Native American jewelry is man-made. The difference can be discerned by the human eye when a magnifier is used to view it. It is much more regular in its color display. Man-made opals also are not as dense as natural opals and may be more susceptible to drying out due to their porosity.
There is nothing inherently wrong with man-made opal, although at one point the Indian Arts and Crafts Association forbade its use in jewelry displayed for sale at the IACA Wholesale Market.
I believe it has since been permitted but only if clearly identified as man-made. It certainly approximates the flare and flash of natural opal when used in jewelry, and at significantly more affordable prices.If you are considering purchasing jewelry with opal cabochons or inlay, be sure to ask the seller if the opal is natural or man-made. Don't pay for natural opal if you are getting man-made opal.
Opal is the birthstone for people born in October, under the sign of Scorpio.