How To Judge Opal

Color

First of all, define the type of opal (black, semi-black, crystal, white, etc.) you are evaluating.

Questions to ask regarding color:

- Play-of-color and pattern of color (if applicable). What are the predominant colors? Reds, violets and oranges are more valuable than greens and blues. Red is the rarest, and if present, all other colors are possible. If blue is visible face-up, then other colors will not be possible. Also take into account the distribution, contrast and abundance of colors present.

- Grading opal patterns is quite subjective and may depend largely of the viewer's personal taste. Generally the more colors that appear to the viewer the more valuable the stone. Stones showing largely blue flash are least valuable.

-Pinfire, consisting of small dots of color is quite common, and not especially valuable, even if the predominant color is red.

-Flash refers to opals with color patterns that appear suddenly as the stone is moved.

-Harlequin refers to opals with a regular arrangement of similarly-sized flashes of color. The more regular the arrangement, the better. Stones showing the full range of colors in this pattern are exceedingly rare and valuable.

- Extinction : Does the full color range appear over a small viewing angle?

- In other words, is there an angle of extinction, during a 360x rotation, no colors are visible? Another way of describing this is directionality - the most valuable stones do not show any extinction of color.

- Brilliancy : How intense are the colors? Do they stand out from the background? How is the color distributed?

- Consider the amount of potch in comparison with the amount of play-of-color. Although potch provides a background for the play-of-color, we buy opal per carat, and so do not want to purchase opals with excessive amounts of potch (potch to play-of-color ratio).

- Sequence of colors : Red that holds over a wide range of viewing angles is the most desirable. In black opal, a red immediately followed by a royal blue is the rarest and most sought-after color sequence.

Practical Tip: Place the opal on a flat surface at arm's length; then rotate it through 360˚ noting the play-of-color, brilliance, and intensity of colors, symmetry and polish. Then hold the opal up to the light and estimate the ratio of potch to play-of-color.

Fine opals should show:

- Uniform play-of-color through 360º
- Intense, brilliant colors
- No 'extinction'
- Symmetrical cut, and fine degree of polish

Judge other opals from this ideal standpoint.

Clarity

Note the following:

- How transparent is the material? High transparency is sought after in light opal, but may reduce the value of black opal.

- Are any fragments of matrix present? Potch, fragments of host rock, sand or other impurities should not interrupt the play-of-color. Material should be free from blemishes visible from top.

- Are there any cracks and flaws visible? Precious opal can contain around 6-10% water and some opals from certain areas may craze, crack and/or turn opaque. Remember, never buy a cracked opal. The crack will only get worse as time passes.

Cut

In judging cut, look for the symmetry (proportions) and polish (finish). Is the make (cut and proportions) of the cabochon good? How good is the surface polish?