By Staff
I know, I'm risking taking this gemstone thing to tedium but there's one more I'm compelled to cover as it's perhaps the most popular of all gems. Few of nature's creations are as intriguing as the opal. All of nature's splendor emanates from the opal, fire, lightning, rainbows, northern lights, blue skies and azure oceans. The fiery embers of color that glow within the opal has captured our hearts for centuries. Despite its magnificence the opal doesn't reach precious gem status because of its fragility. Opal only ranks 5.5 on the hardness scale compared to 7.5, 9 and 10 of the precious gems (emeralds, sapphires/rubies and diamonds, respectively). Opals are also very brittle and easily shattered. Even rapid temperature changes like exposing a cold stone to hot water can break it.
Despite these and some other drawbacks we'll get to in a bit, opal is undeniably the king of semi-precious gems. The opal we are most familiar with is a translucent, milky white with specs of green, orange, red and perhaps violet and blue "fire" sparkling within. This is the most common gem opal and the least expensive. That is far from the only cloak this stone wears, though. The most desirable and expensive is black opal. The black color makes a stunning background as well as enhances the predominantly neon blue fire that it contains. Just slightly behind black in desirability is clear opal, which is as transparent as glass and displays fire in all the colors of the rainbow. I use the term "fire" to describe the flakes of color within the stone. This is not to be mistaken for fire opal. Fire opal is a brilliant yellow, orange or red color and does not display the internal sparkles. Opal also comes in a variety of other colors such as green, blue and even brown. These, too, rarely possess the internal color flakes.
Opal is comprised largely of silicone, making it essentially a form of glass. Oddly, it also contains a certain amount of water, usually in quantities from 3 percent to 10 percent. I was told by my lapidary teacher, Bill Smith, that it was this water acting as a prism to separate the light waves that caused the fire-like flecks within the stone. This was the commonly accepted notion until the 1960s when someone scrutinized opal with an electron microscope. It was found the fire is caused by clusters of closely packed micro spheres of silica gel which diffract the light waves.
Opal is deposited at relatively low temperatures and is typically found filling fissures in other stones. It can also form or replace fossils and is often the component of petrified (fossilized) wood. Opal was known and prized by Old World humans but gem quality material from those parts was rare. It wasn't until Australia's bountiful opal deposits were discovered that opal came into its own. Today Australia provides 95 percent of the world's opal. The remaining 5 percent comes from Mexico (the main supplier of fire opal), Brazil, Idaho and Nevada. The largest black opal in the Smithsonian came from Nevada. Recently some opal deposits have been discovered in Africa.
Opal often occurs in thin sheets not thick enough for a gem stone. It has long been common practice to glue another stone such as black obsidian to the back of the thin opal. Called a doublet, these are very attractive. Check closely before paying big bucks for a black opal to be sure it isn't a doublet. To enhance durability a thin, clear stone such as quartz is sometimes glued to the front of the doublet, making it a triplet. Opal has been referred to as "the stone that begs to be worn." This is because when left lying in a jewelry box for long periods it dries out. This causes it to lose its luster and sparkle and develop cracks. Frequent wearing exposes it to moisture and body oils that prevent this. In fact, my mentor Bill Smith was fanatical about keeping opals, both raw and cut, stored in water or mineral oil. Okay ladies, there's your excuse to wear those opal earrings you got for Valentine's Day. Carpe diem.