Opal ClassificationIndividual opals can vary widely in appearance and quality. As diverse as snowflakes or fingerprints, each gem can differ noticeably.
There are three main aspects of an opal’s quality:
- Color—Background color and play-of-color
- Pattern—Arrangement of play-of-color
- Clarity—Transparency and quantity of inclusions
Play-of-color might be the most spectacular aspect of an opal’s appearance. No matter the color or combination of colors, play-of-color must be vivid to command a high rating. In other words, opal lovers prize bright play-of-color over faint play-of-color.
Color - 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: 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
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.
The brightness of an opal is measured on a scale from 1 to 5. 1 being dull and 5 being brilliant. Only the brightest of gems deserve a 5, while most opals are between 3 and 4.5. When buying an opal, make sure the brightness is 3 or more on the scale.
The body tone of an opal is different to the play-of-colour displayed by precious opal. Body tone refers to the relative darkness or lightness of the opal, while ignoring its play-of-colour. This is assessed on a Scale of Body Tone . The boxes (below) comprising this scale, represent approximate values of body tone in equal intervals from black to white.This arrangement is in agreement with all known scales of tone used in colour science, and is well illustrated in the commercially available Rock-color Chart produced by the Geological Society of America.
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