Opals are in a class by themselves. As a species, opal is so unique it has its own descriptive vocabulary. More than any other gem, each opal is distinctly individual. Opals are also the most delicate gemstones commonly worn and require special care.
What’s the Difference Between “Common Opals” and “Precious Opals”?
Opal is an amorphous form of silica, chemically similar to quartz, but containing 3% to 21% water within its mineral structure. Gem grade opals usually have 6% to 10% water content.
Opal is a sedimentary stone. Under the proper conditions, water percolates through the earth, becoming rich in dissolved silicates. When water enters a cavity, it deposits the silicates as microscopic spheres, forming opals.
If the spheres are uniform in size and shape and neatly stacked, they will diffract light. These stones are called precious opals. If the spheres are random in size, shape, and arrangement, the results are common opals. Common opals can have an opaque or glassy appearance with a waxy luster. Seldom cut, these stones come in a wide range of colors. Common opals are often fluorescent. Precious opals, also known as “noble opals,” display fire or play of colors.
Opal’s characteristic fire, or play of colors, was long thought to be the result of iridescence. However, with the advent of scanning electron microscopes, we now know it’s a result of diffraction. This phenomenon of flashing or moving colors due to diffraction isn’t related to the body color of the opal.
The particular colors seen in an opal’s fire depend on the size of the spheres and the angle of viewing. For example, black opal gets its color from volcanic ash, but inclusions have nothing to do with the play of color. That is due entirely to the tiny silicate spheres. They must be smaller than 1,500 angstroms (Å) for blue and violet colors, but no larger than 3,500 Å to produce oranges and reds. To put that in perspective, 20,000 spheres are about the size of the period at the end of this sentence. (An angstrom is one ten-billionth of a meter).
Opal Varieties
Black Opal
Black opal is characterized by a dark body tone which can range from dark grey to jet black. However this refers only to the general body tone of the stone, and is not related to the rainbow or spectral colours present in the opal. Some people expect a black opal to be completely black (in which case it would be completely worthless).
Unlike ordinary opals, black opals have carbon and iron oxide trace elements present, which cause the unusual darkness of the stone. Because of their dark body tone, the rainbow colours in a black opal stand out much better than lighter opals. This vibrancy of colour makes black opal the most valuable form of all opals. Often a black opal will have a natural layer of potch (colourless opal) left on the back of the stone, which can give the stone an extra darkness and vibrancy of colour. The darker this potch backing is, the darker the body tone, which usually makes for greater vibrancy of colour and a higher value black opal. This is the principle upon which doublets and triplets are based, which are an imitation of solid black opals.
Black opals are generally cut into an oval shape, however this depends on the natural shape of the stone. Sometimes black opals are cut into freeform or teardrop shapes in order to maximise the size and carat weight of the stone. Black opals are normally cut with a low cabochon, due to the opal colour bar often being much thinner in black opal than in white or crystal opals. If the colour bar in an opal is thin, it is impossible to create a domed surface.
Top of the range gem quality black opal can fetch prices up to $15,000 per carat. However, just because an opal is black doesn’t make it valuable. There are many factors including brightness and pattern which determine the overall value of opal.
Fire Opals
Translucent to transparent with a yellow, orange, or red body color, these stones may or may not display a play of colors. The “fire” in their name refers to their body color, not to play of color. These stones are also called Mexican or sun opals.
Boulder Opal
Thin seams of opal that form in ironstone, these gems come in many colors and show dazzling fire, backed by their brown ironstone matrix.
Common opal occurs in seams of rock outcroppings north of Princeton, and also in tertiary rocks at Savona Mountain, Agate Mountain, Horse Fly River, Fourmile Creek, and Slocan Lake. Good quality hyolite is found near Hihum Lake and the Bonaparte River. Along the banks of the Deadman Creek, fire opal has been found. Well-grained opalized wood in black, brown, white, and green hues have been found along Barnes Creek near Ashcroft. Precious opal occurs only in a few locations in B.C.
The Eagle Creek deposits near Burns Lake have been set aside for rockhounders. Milky coloured opals with only small portions displaying a red and green play of colour have been found on an unnamed mountain west of Penticton. Most recently several interests are attempting to engage in a commercially viable mining of opal in the Vernon area at the Klinker deposit.
