The splendid iridescence of shimmering colors, rippling over their surface, makes the opal a mysterious, elegant stone that can be as costly as a diamond’ – Cartier ‘Cartier Royal High Jewelry and Precious Objects’ 2014

     Lightning Ridge black opal is the rarest, most precious, and most valuable form of opal. Its unique play of varying colors, brilliance, and pattern also make it the most difficult gem to judge. It cannot be commoditized just as any one work of art cannot be, because its appreciation varies with every individual who views it.

     However, the 4 valuation factors of body background, colour, brilliance and pattern provide a base system built upon their rarity in nature and their beauty, which provides a guide to ascertain the quality and thus relative value of an opal.

     Each factor has a different weight on the final price. Body background constitutes 15%, Colour 35%, Brilliance 25%, and Pattern 25%.

Body Background

A 7.01 carat blue-green gem with a perfect black tone

     The darkness of a black opal is determined by the tone of the background of the stone. Top rating for darkness might be the black of a midnight sky, and, as the background tone decreases until it is a light colour, so does the relative value of the stone.

Colour

This 14.77 ct black opal contains large amounts of desirable red colouration

     Black Opals containing a predominant red colouration have the highest value, then orange, gold, green, and finally blue. The tone of the colour is also very important and anyone can see the difference between a pigeon blood ruby red and a light pinkish red.

     Value is added to include the prominent colours in the stone and the various shades as to how true or dominant they might be.

Brillance

A 15.50 ct blue, green, and orange gem that exhibits tremendous luster.

     We often refer to it as the luster of the colour, as in how bright or luminous it appears. If you look at the stone from a distance and the colour is jumping out at you then it has a high luster, conversely if the colour appears dull then it has a low luster.

Pattern

This 47.10 ct gem exhibits the rarest of the rare,
the harlequin pattern with red colouration.

     If the pattern captivates you then it gets a higher valuation. The broader patterns attain higher values. The rarest and most highly prized pattern is the harlequin pattern. Other noted patterns are chaff, Chinese writing, floral, straw, mackerel, ribbon, rolling flash, flagstone, and broad flash.

Deductions

As a rough guide an opals value is reduced if certain factors exist:

  • The flatter the surface of the stone the less value it will have.
  • If it doesn’t look equally good in all lighting – sunlight, artificial light, and the shade.
  • If it has intrusive potch lines that interfere with the pattern and play of colour.
  • If it has excessive sand in the background
  • If it has excessive potch on the back of the stone that adds to the carat weight
  • If the stone has a window where no colour appears on the face of the stone.
  • If it is an off shape or free form