Opal

 Opal, more than any other gemstone, is distinctly an individual. No other stone has as rich and varied a folklore. Precious opal displays rainbow-like hues that change with the angle of observation, especially in rounded cabochon forms. It was only in the 1960s that a team of Australian scientists analyzed Opals with an electron microscope and discovered the cause of opal’s unique optical properties. It was discovered that small spheres from silica gel caused interference and refraction, which are responsible for the fantastic play of colors. The spheres, which are arranged in more or less compact structures, dissect the light on its passage through the stone.

Opal by definition is hydrated silicon dioxide, always containing from three to thirty percent water. That characteristic and its relative softness with a rating of 5.5 – 6.5, means that opals require some special care.

Opal is a birthstone for those who are born in October.

Opal colors
Color is white, colorless, pale yellow, pale red, gray or black when impurities are common. Diffraction can cause flashes of any color of the rainbow (opalescent). That effect is so fascinating that it deserves a more lengthy description: As light enters the opal, it bends around the edges of tiny particles of hydrated silica, “chips” of silicon and oxygen suspended in water within the stone. When it is diffracted, the light that is made up of all visible colors, each with its own wavelength, produces an entire rainbow of colors.

Precious opal display rainbow-like hues that change with the angle of observation, especially in rounded cut forms.

Black opal, precious opal with black body color.

Semi black, or gray opal, precious opal with dark body color.

White opal, precious opal with white or very light body color.
Crystal opal, transparent to semitransparent, colorless body with play of color.

Black crystal opal, transparent to semitransparent opal, with dark body color and play of color.

Fire opal, translucent to transparent, with yellow, orange, or red body color.  May or may not have play of color. Also called Mexican opal or Sun opal.

Harlequin opal, transparent to translucent precious opal with effective mosaic-like color patterns. Counted among the most desirable opals.

Jelly opal, bluish-gray precious opal with little play of colors.

Boulder opal, a thin seam of precious opal on ironstone matrix. Since this is a natural occurrence, its value is higher than that of a man made doublet. The ironstone is very dark, which makes the fire stand out and gives a close resemblance to black opal.

Matrix opal consists of thin seams or spots of precious opal in matrix. The matrix is usually dark ironstone, but occasionally a light sand stone.

Common opal, also called potch, is opaque, rarely translucent and shows no play of color. A wide variety of trade names are used to identify common opals.

Agate opal, agate with light and dark opal layers.
Angel skin opal, misleading name for palygorskite, an opaque, whitish- to pink-colored silicate mineral.
Girasol, almost colorless, transparent opal with bluish opalescence
Honey opal, honey-yellow translucent opal.
Hyalite, glass opal or water stone, colorless, water-clear opal with strong sheen.
Hydrophane, a milk opal, which has turned turbid due to the loss of water. Through absorption of water, it can become translucent again and have color play.
Porcelain opal, white, opaque milk opal.
Prase opal, apple-green opal
Moss opal, milk opal with dendrites.
Wax opal, yellow-brown opal with wax-like luster.
Wood opal, yellowish or brownish opal as fossilized wood.

Doublets and Triplets make use of opal that is too thin to use as a solid gemstone. A doublet is a thin layer of precious opal glued to a black base. A triplet adds a transparent, quartz cap. Purists prefer the base material to be common opal. However many black materials are used, including old phonograph records!

Play of color, body color and transparency are the criteria which determine the price of a precious opal.

Opals are delicate gemstones. Their most significant weakness has to do with its water content. If an opal is allowed to dry, it will crack and fade. If you live in a very dry climate, or keep them in a dehumidified room, some precautions are necessary. Keeping them in a tight plastic bag, with a damp piece of cotton or fabric will prevent dehydration. Because of their water content, opals are also highly sensitive to sudden changes in temperature.
Being somewhat soft, they scratch easily. Realize that a large component of ordinary dust is quartz at 7 (Mohs scale) in hardness. At 5.5 to 6 in hardness, simply wiping the dust off an opal will gradually reduce its polish. The solution is to clean your opals using a soft cloth.

Opals are best suited for earrings, brooches, and pendants.

Color
The play of color and the saturation of the body color mark the difference.

Lighting
Opal looks best viewed in daylight (just after sunrise and before sunset), which exposes its exceptional color play most favorable that changes with the angle of observation, especially in rounded cut forms.

Clarity
Precious opal designed its own adjective for clarity, opaque. Some rare specimens are transparent to translucent.

Cut
Mostly round or cabochon cut. Fine quality fire opals are facetted.

Opal location and deposits
The most important deposits are located in Australia. Around 95 percent of the world’s supply of white precious opals is mined “down under”. Famous deposits in New South Wales are at Lightning Ridge and White Cliffs, in South Australia at Coober Peddy and Andamooka. Various deposits are also found in Queensland.

Other deposits are found in Brazil, Guatemala, Honduras, Indonesia, Japan, Mexico, Russia and the United States (Idaho and Nevada).

Common opal treatments
Opal is often impregnated with oil, wax, or plastic.

World-famous Opal
“Big Ben”, a white precious opal of over 800 grams, more than 4000 carats, was found at Lightning Ridge.

“The light of the world”, a white precious opal of 40 grams, 2,250 carats, found in Australia as well, is considered to be the most colorful white precious opal of all times.

“Andamooka Desert Flame”, with a weight of 6,843 kilograms the largest opal ever brought to the light of day.

Opal gemology
Species: Opal
Color: All colors, partially play of color
Chemical composition: SiO2 .nH2O hydrous silicon dioxide
Crystal system: Amorphous; kidney- or grape-shaped aggregates
Hardness: 5.5-6.5 (Mohs scale)
Specific gravity: 1.98 – 2.50
Refractive index: 1.37 – 1.52
Birefringence: None
Optical character:
Color of streak: White,
Absorption spectrum: Fire opal: 700 – 640, 590 – 400
Fluorescence: White opal: white, bluish, brownish, greenish; Fire opal: greenish to brown

The Opal zodiac, myth & legend
The name opal was probably derived from Sanskrit “upala”, meaning “valuable stone”. This was most likely the root for the Greek term “opallios”, which translates as “color change”. In the days of Roman antiquity there existed a so-called “opalus”, or a “stone from several elements”.

Pliny, the famous Roman author, called Opal a gemstone which combines the best possible characteristics of the most beautiful of gemstones: the fine sparkle of Almandine, the shining purple of Amethyst, the golden yellow of Topaz, and the deep blue of Sapphire, “so that all colors shine and sparkle together in a beautiful combination”.

According to the legends of he Australian Aborigines, the creator came down to Earth on a rainbow, in order to bring the message of peace to all the humans. And at the very spot, where his foot touched the ground, the stones became alive and started sparkling in all the colors of the rainbow. That was the birth of the opals.

For ages people have believed in the healing power of Opal. It is reported to be able to solve depressions and to help its wearer find the true and real love.