Opal Glassware

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.
Opals are often divided into types based on background color. Even though there are many different categories for opals, here are the main five types:
Black Opal: All other quality factors being equal, many buyers favor the dark background color of black opal. This is partly because play-of-color tends to stand out attractively against a dark background. The contrast of play-of-color to bodycolor makes black opals very popular. Additionally, black opals are considered to be the rarest (white opals are more common).
White Opal: An opal with a translucent to opaque white and other light color backgrounds (bodycolor) with play-of-color is called white opal.
Fire Opal: Fire opal is transparent to translucent with a bodycolor that is usually yellow, orange or red. This material, which might show play-of-color, is also known in the trade as "Mexican opal" or "Mexican fire opal".
Boulder Opal: Boulder opal is translucent to opaque opal with play-of-color within a host rock. Thin layers of opal exist within the host rock (called matrix). The opal is cut with the matrix attached and is part of the finished gem.
Crystal and Water Opal: Crystal opal is transparent to semitransparent with a clear background. This type of opal can show excellent play-of-color. Water opal might or might not display play-of-color. If it does show play-of-color, it is faint and covers only small portions of the gem.
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.
Secondary in importance to play-of-color's strength is its range. If an opal's play-of-color is not just bright, but also ranges across the entire spectrum, it's very rare and valuable. Not every precious opal, however, sparkles with every color of the rainbow. In some, the play-of-color consists of just one main color and two or more secondary colors.
Desirable play-of-color is further broken down by the colors themselves. Traditionally, red is considered the best prominent color, orange the next most desirable, followed by green. However, favored colors can vary with fashion or personal preference.
In addition, an opal's play-of-color can change along with the viewing angle or type of light. For example, red might dominate in the same portion of an opal cabochon where blue dominates when it's viewed from a different angle.

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
In addition to the arrangement and shape of the play-of-color patches, buyers must consider extinction, or "dead spots," when evaluating pattern. A dead spot is an area of the gem in which no play-of-color appears and only background color is visible. Dead spots detract from opal value, especially if there are several of them.

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.
There are various types of opal clarity characteristics that affect value. Opals, like other gems, can have fractures, or pits and other surface blemishes. An opal might also contain fragments of its host rock, called matrix. Matrix in a polished opal is usually-but not always-detrimental to its appearance and value. It depends on the type of opal.
If an opal loses moisture, it can lead to crazing: A fine network of cracks that resembles a spider's web. The moisture loss can be caused by heat or excessive dryness, or by exposure to bright light or direct sunlight. Crazing can be prevented by never displaying opals in places- such as window displays-where they're exposed to these conditions.