OPAL

Opal is the world's most popular phenomenal gem. Many cultures have credited opal with supernatural origins and powers. Arabic legends say it falls from the heavens in flashes of lightning.

The ancient Greeks believed opals gave their owners the gift of prophecy and guarded them from disease. Europeans have long considered the gem a symbol of hope, purity, and truth.

Opal is one of the US birthstones for October (along with tourmaline). Some people think it's unlucky for anyone born in another month to wear an opal, but that particular superstition comes from a novel written in the 1800s (Anne of Geierstein by Sir Walter Scott) and not from ancient belief or experience. In fact, throughout most of history, opal has been regarded as the luckiest and most magical of all gems because it can show all colors. Once, it was thought to have the power to preserve the life and color of blond hair.

Although experts divide gem opals into many different categories, the main types are:

White opal - translucent to semi-translucent with play-of-color against a white or light gray body color.
Black opal - translucent to opaque with play-of-color against a black or other dark body color.
Fire Opal - transparent to translucent with brown, yellow, orange, or red body color. This material, which often does not show play-of-color, is also known as "Mexican opal," "gold opal," or "sun opal."
The market supply of fine black opal is extremely limited, but white and fire opals are generally available in a wide range of sizes. You'll usually see black or white opals fashioned as cabochons and set in rings, pendants, pins, or earrings. Fire opals are used in the same kinds of jewelry, but they're often faceted. All three types occasionally appear as beads and carvings.

Pieces of white or black opal that are too thin to use alone often become part of opal doublets or triplets. In these assembled stones, a sliver of opal is cemented, usually with black adhesive that dramatizes the play-of-color, to a backing such as chalcedony, glass, or plastic. A doublet consists of two pieces (the opal and the backing), while a triplet also has a protective top made of rock crystal quartz or colorless glass.

Explaining Play-of-Color
Play-of-color occurs because opal is made up of sub-microscopic spheres stacked in a grid-like pattern, like layers of ping-pong balls in a box. This structure breaks up light into spectral colors. The colors you see depend on the sizes of the spheres. Those approximately 0.1 micron (one ten-millionth of a meter) in diameter produce violet. Spheres that are about 0.2 microns in size produce red. Those in between produce intermediate hues.

Common trade terms for play-of-color include:

Pinfire or pinpoint - small, close-set patches of color
Harlequin or mosaic - broad, angular, close-set patches of color
Flame - sweeping reddish bands or streaks that shoot across the stone
Peacock - mainly blue and green

Sources



Australia Black and white opal

Brazil White opal

Mexico Fire opal


Hardness & Toughness

Hardness 5 to 6½ on Moh's scale

Toughness Very poor to fair


Stability
High heat or sudden temperature change can cause fracturing. Opals are generally stable to light, but heat from intense light can cause fracturing (known as "crazing"). Opals are attacked by hydrofluoric acid and caustic alkalis. Loss of moisture, and crazing, can result from storage in airtight containers, such as safe deposit boxes.


Treatments (all treatments listed are common)

Treatment Purpose Stability Detection
Impregnation with oil, wax, or plastic.

Improves play-of-color and prevents or disguises fracturing. Black plastic also creates the appearance of black opal. Fair to poor for oil or wax; excellent for plastic. Oil and wax treatments are detectable by a trained gemologist or gemological laboratory. Advanced laboratory testing is almost always required for plastic. *
Soaking in dye, silver nitrate, or sugar and acid (known as "sugar treatment"). Creates or improves play-of-color and simulates the appearance of black opal. Poor to good Detectable by a trained gemologist or gemological laboratory. *
Smoke impregnation.
Creates or improves play-of-color and simulates the appearance of black opal.

Fair to poor. Treatment is shallow, and abrades or chips away easily. Detectable by a trained gemologist or gemological laboratory. *
* If there is any doubt, send the gem to a gemological laboratory for verification

Care and Cleaning
Opals can be cleaned with warm, soapy water. Avoid ultrasonic and steam cleaning.

Imitations and Synthetics
Glass and plastic have been used to imitate opal, and synthetic opals are available in a variety of colors

Alternatives
No gem duplicates opal's unique combination of color and phenomenon. As alternatives, you might suggest stones with similar body colors, or those that show other special optical effects, such as fire agate, and iris agate.

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