Opal Overview

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. For thousands of years, people have mined and treasured opals. These striking gems have inspired a rich body of folklore. (So rich, in fact, opal gems have been considered both the luckiest and unluckiest stones you can wear). Nevertheless, some scholars believe many ancient references to opal may actually have been to other gems, such as the iridescent iris agate

Occurrence and Mining

Opal is a hydrated amorphous form of silica (SiO2•nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt. There are two broad classes of opal: precious and common. Precious opal displays play-of-color (iridescence), common opal does not.[4] Play-of-color is defined as “a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light.”[5] The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Depending on the conditions in which it formed, opal may be transparent, translucent, or opaque and the background color may be white, black, or nearly any color of the visual spectrum. Black opal is considered to be the rarest, whereas white, gray, and green are the most common.

Quality Factors

Opal hues can range across the spectrum. An opal might display a single color, two or three colors, or all the colors of the rainbow. Opal displays background color in addition to play-of-color. Background color—also called bodycolor—is caused by the suspension of tiny impurities within opal’s silica spheres.
Pattern describes the arrangement of an opal’s play-of-color. Like the shapes you see in the clouds, play-of-color takes many forms. Common terms for play-of-color patterns include: • Pinfire or pinpoint: Small, closely set patches of color • Harlequin or mosaic: Broad, angular, closely set patches of color • Flame: Sweeping reddish bands or streaks that shoot across the stone • Peacock: Mainly blue and green
The cutter considers an opal’s color, pattern, and clarity when planning the finished gem. As with many top-quality colored stones, exceptional opals might not be cut to standard sizes and shapes.
With an opal, clarity is its degree of transparency and freedom from inclusions. An opal’s clarity can range all the way from completely transparent to opaque. Experts prize different levels of clarity for different opal types. For example, in crystal opal, experts admire transparency, while in black opal they prefer an opaque background. Each provides the best background for displaying play-of-color in its individual opal type. A cloudy or milky background color lowers the value of any opal. It makes the gem less attractive, and it can sometimes signal a lack of stability.

Identification

Color

Colorless, white, yellow, red, orange, green, brown, black, blue, pink

Crystal habit

Irregular veins, in masses, in nodules

Cleavage

None

Fracture

Conchoidal to uneven[1]

Mohs scale hardness

5.5–6

Luster

Subvitreous to waxy

Streak

White

Diaphaneity

opaque, translucent, transparent

Specific gravity

2.15+0.08
−0.90[1]

Density

2.09 g/cm3

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