How to grade an Opal

Opals are the most commercially important and phenomenal gems. Opals internal structure interacts with light to create a phenomenon called play-of-colour. Some opals show more play-of-colour than others, and the phenomenon can appear against a dark, light or even transparent background.

The finest opal features all the spectral colours in an intense display that covers its entire surface. Inexpensive opals might have only tiny dots of colour against a dull, opaque background. Between these two extremes is a wide variation in appearance.

When experienced opal dealers go on buying trips, they take sample stones with them for reference. That way, if lighting conditions at the seller’s office are very different from the lighting conditions back home, they have gems of known quality for comparison.

To evaluate an opal, place it face up on a flat surface. A black, matte-finished surface provides the best background for evaluating play-of-colour. Use a standard daylight fluorescent grading lamp positioned about 10in (25 cm) from the stone. Rotate the gem 360 degrees and note background colour, play-of-colour, symmetry and polish. As you rotate the opal, examine the visibility and brilliance of its play-of-colour from different angles.

The first step in opal grading is determining the opals type, which is based on its background colour. There are 4 main opal types: Black, Boulder, and Crystal and White opals. Black opal shows play-of-colour against a black background, Boulder against a brown background, Crystal against a transparent background and White has play-of-colour against white. There are also opals with backgrounds in various shades of grey, but they are less valuable than pure black ones. All other quality factors being equal, the industry values black opal most highly. Its play-of-colour tends to stand out most attractively against the dark background.

After you determine the opals type, you have to judge its play-of-colour. Try to determine the dominant phenomenal colours. Fine opal should show the full spectral range – red, orange, yellow, green, blue and violet. If there’s just one colour red tends to be the most valuable. Opals showing mainly greens and blues are less valuable than opals showing red, yellow and orange.

The colours should be bright and intense, not dull and muted, and you should be able to see them at a normal viewing distance, and from many angles. In some opals, the flash of phenomenal colour is only seen from one small viewing angle. These gems are much less valuable than opals that show play-of-colour from all angles.

Play-of-colour should cover the opals entire surface. Patches without phenomenal colours reduce an opals value, especially if there’s more background colour than spectral colour.

Another play-of-colour consideration is the pattern. In general, opals show three types of patterns. Pin fire patterns feature very small patches or dots of play-of-colour, flash patterns consists of large area’s of play-of-colour and harlequin pattern have large, distinct patches that are usually angular, with edges that touch. Harlequin patterns in a full range of hues are extremely rare and valuable. The next grading step is to determine the opals clarity and transparency. Judge it only against other opals of the same type. High transparency is desirable in crystal opal, but tends to reduce the value of black opal. Examine the gem for pits and blemishes. If they break the opals top surface or effect play-of-colour they greatly reduce value. Check for signs of poor durability like cracks, flaws, and cloudy areas. An opal cabochon should exhibit good quality surface polish, with no dull spots.

You should also check the symmetry of the opals cut. A domed surface gives the best play-of-colour, and results in a vivid appearance from most viewing angles. The dome should be fairly shallow, with a gentle curve. The opal used in commercial quality jewellery is commonly fairly flat.

Finally examine the opal for networks of fine cracks, called crazing. Crazing reduces an opals value considerably.

The following is a useful summary to follow when grading an opal you are considering buying:

1. Determine the opal type: Black, Boulder, Crystal or White opal
2. Evaluate the play-of-colour. Determine its intensity and hue range, and how much there is of it compared with the opals background colour. Determine if it appears over a variety of viewing angles.
3. Determine the pattern of the play-of-colour. Check to see if it’s present in small specks (pin fire), large flashes (flash), or angular patches (harlequin).
4. Evaluate the opals clarity. Judge its transparency and inclusions, and examine it for crazing.
5. Evaluate the opals cut. Examine the cabochon’s height, shape and symmetry.