Opal Quality Factors
There are three main aspects of an opal’s quality:
- Colour—Background colour and play-of-colour
- Pattern—Arrangement of play-of-colour
- Clarity—Transparency and quantity of inclusions
Opal hues can range across the spectrum. An opal might display a single colour, two or three colours, or all the colours of the rainbow. Opal displays background colour in addition to play-of-colour. Background colour—also called bodycolour—is caused by the suspension of tiny impurities within opal’s silica spheres.
Opals are often divided into types based on background colour. Some background colours tend to be more prized than others. All other quality factors being equal, many buyers favour the black background colour. This is partly because play-of-colour tends to stand out attractively against a dark background.
Play-of-colour might be the most spectacular aspect of an opal’s appearance. Regardless of the colour or combination of colours, play-of-colour must be vivid to command a high rating. In other words, opal lovers prize bright play-of-colour over faint play-of-colour.
Secondary in importance to play-of-colour’s strength is its range. If an opal’s play-of-colour 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 colour of the rainbow. In some, the play-of-colour consists of just one main colour and two or more secondary colours.
Desirable play-of-colour is further broken down by the colours themselves. Traditionally, red is considered the best prominent colour, orange the next most desirable, followed by green. However, favoured colours can vary with fashion or personal preference.
In addition, an opal’s play-of-colour 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.
The most valuable opals display play-of-colour from all angles.
Pattern describes the arrangement of an opal’s play-of-colour. Like the shapes you see in the clouds, play-of-colour takes many forms.
Common terms for play-of-colour patterns include:
- Pinfire or pinpoint: Small, closely set patches of colour
- Harlequin or mosaic: Broad, angular, closely set patches of colour
- Flame: Sweeping reddish bands or streaks that shoot across the stone
- Peacock: Mainly blue and green
In general, connoisseurs prefer large, closely arranged patches of colour over tiny, scattered dots. As with any play-of-colour, no matter what the pattern, colours must be bright for the stone to be valuable.
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-colour appears and only background colour is visible. Dead spots detract from opal value, especially if there are several of them.
Clarity and transparency
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-colour in its individual opal type. A cloudy or milky background colour 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.
Even if cracked opals don’t break right away, they have little durability in jewellery, and the fractures spoil the beauty and clarity of the gem. For quality control, producers and dealers single out opals that they suspect might ultimately craze. Dealers who willingly buy crazed opal do so because it commands a lower wholesale price than undamaged opal. This means they can sell it at a lower retail price level.
The cutter considers an opal’s colour, pattern, and clarity when planning the finished gem. As with many top-quality coloured stones, exceptional opals might not be cut to standard sizes and shapes.
Cutters usually fashion exceptional rubies or sapphires, for example, in a way that saves weight or maximises colour— even if the result is an unevenly proportioned gem. In a similar strategy, opal cutters usually cut top-quality rough to show off its spectacular play-of-colour.
To achieve this goal, cutters might fashion fine opals into large, irregular shapes that keep as much play-of-colour as possible. Designers set these costly one-of-a-kind gems into custom pieces.
On the other hand, cutters typically fashion commercial-quality white opal and assembled opal into standard calibrated sizes, usually ovals.
The cut of a fine opal should be symmetrical. If it’s a cabochon, the dome should be well rounded. Domed surfaces give the best play-of-colour, and make the stone appear vivid from most viewing angles. If the cabochon is flat, it might be vulnerable to breakage, especially during setting into jewellery. If it’s too high, it might be hard to set in jewellery.
Opals come in a wide range of sizes and carat weights. Opal has relatively low density compared to many other gemstones so even larger sizes can be comfortable to wear.
Common sizes for many of the opal cabochons set in jewellery are 6×4, 7×5, and 8×6 mm.
Opal can be fragile. In solid opal cabochons, the gem material is usually thick enough to withstand everyday wear and jewellery repair without breaking. But with thinner material, manufacturers often have to add a rigid backing for durability.
Assembled opals are fashioned opals with backings. They might include materials like glass that aren’t usually part of gem-quality jewellery, but because they are partly precious opal, they still have value as gems. They sell for only a fraction of the price of boulder opals, but they allow manufacturers to make attractive finished gems from thin opal pieces.
The two common types of assembled opal are the opal doublet and triplet. The doublet is a thin layer of opal cemented to a backing. The backing is often composed of obsidian, dyed black chalcedony, black glass, natural common opal or plastic. The triplet is a thin layer of opal cemented between a domed top of colourless quartz or clear glass and a backing of obsidian, chalcedony or black glass.