Opals are a wonderful gem because the individual opal has all the colors of the rainbow. The inclusions are described as flashes, sparks, or flames of light, playing along the surface of the opal. The finest variety of opal, so called the Harlequin Opal, has multicolored flakes throughout the gemstone; while, the Mexican Opal are more transparent with patches of coloring. The Fire Opal that also comes from Mexico has but one color, a fire-like red. Unfortunately, the range of "fire-like" red merges into yellow. Opals are mined soft and brittle then harden due to exposure. Even though the most hardened opal may still chip, many of them are cut and facet for settings.
Opals are a type of silicate that contains molecules of water within its structure. Due to this hydration, the opal is not considered to be a mineral nor a crystal. Opal is an amorphous solid. They are gems as their name in Sanskrit suggests and are divided into classes based on the inclusion of iridescence, namely the common opal do not display iridescence. Opals can be opaque, transparent, or translucent — can be all sorts of various colors -- so the interplay of the two types, precious and common, determines its value. But, how long lasting is the effect of iridescence in a precious opal? Of the variety of opals, which is in fact the most precious?
Diffraction which occurs inside the opal should not be confused with opalescence. Maybe one way to explain the difference between the two properties would be to compare the light passing through the opal against the light reflecting off its surface. For the sake of argument, iridescence is an intensification of light.
The more valuable opals are opaque and iridescent, and the most valuable cut for an opal is the cabochon. Cabochons can be semi-spherical displaying the optical effect of a gemstone optimally. With this information, the cut (cabochon), color (iridescent), and clarity (opaque) are met giving the owner of a precious opal the freedom to make friends. Incidentally, opal is the birthstone for those with the astrological symbol libra.
Ceteris paribus, the background color is the preference of the owner. However, gems especially precious gems were believed to be the home of spirits. The opal, according to George F Kunz, for centuries was thought to combine every virtue united under the rainbow so was feared to be worn as jewelry. In his opinion, the superstitions given to gemstones prevented an appreciation for their aesthetic beauty alone. For opals timid people would not wear them.
“The eye of some invisible monster, the eye of the dragon, the eye of the serpent, were all regarded as possessed of malign power.” quote from George F Kunz
Opals gained the bad reputation for being unlucky during the Modern Era based upon a reading of the 1829 book “The Maiden Of The Mist” by Sir Walter Scott. Lady Hermione, the book’s antagonist, wears a large opal in her hair which corresponds with her power. However, after becoming wetted by holy water, both the opal and Lady Hermione turn to dust. Water broke the spell. Opals from antiquity have proven to be difficult to find because over a century, depending on where the opals were mined, they tend to lose their water content. Fire opals have been known to turn transparent within a few years. And, the iridescence of an opal can also be found in common quartz, namely “iris quartz”, confusing the issue of water content more so. According to Pliny the Elder, opals were substituted by mineral quartz. The situation in antiquity was unfairly against the opal and none seem to have survived the course of time.
Of all precious stones it is the opal that presents the greatest difficulties of description, it displaying at once the piercing fire of carbunculus, the purple brilliancy of amethystos and the sea green of smaragdus, the whole blended together and refulgent with a brightness that is quite incredible. —Pliny the Elder
Black opal regarded as a gemstone of good luck is known to be made artificially by either dying the opal in black ink or heat fracturing then filling the cracks with burnt oil. And, suddenly, a natural deposit of black opal was discovered in New South Wales.
Opals are more valuable en cabochon, opaque, and iridescent; yet they can lose their iridescence over a century or few years depending on where they were mined. During the Middle Ages, opals were mined at Cernowitz, Hungary. Today the Slovak Mines are open to tourism and stretch underground for 35 kilometers. Hungarian opals were worn by women as necklaces or used as hair pins. Opals were known in Scandinavia as the yarkatein. Queen Elizabeth noted that the superstition against wearing opal had some connection with the Evil Eye, a protective talisman. Opals were certainly used as eye inserts for statuary at some time, and the belief emerged that wearing an opal would turn a person invisible. Whether this occurred after wetting or drying the stone is not known. In conclusion, opals are not risk-free at the point of view of the Modern Era.