Valuing Opal
How rare is opal?
Silica is one of the most common minerals on the planet, but precious opal composed of an orderly three dimensional array of silica dioxide spheres is very rare – far scarcer than diamonds. Precious gem opal is rare because the very specific natural processes that created it are confined to a very small part of the world.
Most (at least 95%) of the opal found by miners is potch opal without gem colour. Potch can be white, grey, black or amber coloured. Even when a miner finds gem-coloured opal, most of it can’t be cut into gemstones because it’s too thin, or sandy.
Opal is not only incredibly rare, but it is mathematically impossible to have two stones alike given that there are over 50 billion silica spheres in each cubic millimetre of opal!
What is the methodology utilised to assess and value and opal?
There is a formal “nomenclature” for all gemstones including opal, which is a way to describe a specific gemstone in descriptive words.
Opal valuation follows a methodology of considering many characteristics from the nomenclature including opal type, body tone, colour, pattern, brightness, provenance, diaphaneity, weight, shape & cut.
Each characteristic is considered according to its rarity and/or influence and an overall value per carat (5 carats = 1 gram) is determined, then the stone is weighed. This applies to finished gems as rough can be sold by gram or carat subject to where it is mined.
Market dynamics will also come into play similar to buying real estate i.e. an opal sold in Lightning Ridge will be significantly less expensive than the same opal in Sydney, Tokyo or New York.
How do you put a dollar value opal?
Fundamentally we follow the objective nature of the diamond grading scale (colour, cut, clarity, carat weight), with specific or additional significance given to a combination of supplementary factors:
- Brightness: (brightest is best)
- Colour: (purple is the most common colour, then blue, then green, through gold then orange to red, which is the most rare and therefore valued most highly)
- Pattern: (some patterns like Script or Harlequin are more rare than others, like Pinfire for example)
- Body tone: (the body tone is the darkness of the opal if you ignore its colour; all things being equal the darker the body tone, the more rare and therefore more valuable the opal)
- Consistency (play of colour): for example, an opal that looks bright and beautiful from all directions is valued more highly than one that looks fantastic when held in one position, but dull in another position.
- Provenance: Where does the opal come from? Lightning Ridge is the ‘home of black opal’ so having this provenance adds significant value to an opal.
- Shape and cut: Calibrated shape (perfect proportions), free shape (irregular proportions), scratch free mirror finish, thickness of colour dome.
Other factors include whether there is any sand or potch showing on the face of the opal.
Opal valuation boils down to rarity: the rarer the combination of features possessed by an opal, the more highly it is valued. And like fine art or fine wine or other fine jewels, final price may also be influenced by personal taste and market conditions.
The basic unit of valuation is the same as for other gemstones – a weight unit called a carat – opals are priced per carat.
Vicki Bokros – owner and founder of DTEO has been in the opal business since 1991. She has personally mined opal in Lightning Ridge, travelled the world with opal and has vast experience in export, wholesale and retail of the product.
Vicki is a qualified gemmologist, member of the International Colored Gemstone Association (ICA), Gemmological Association of Australia (GAA), Opal Association of Australia (OA) and one of a very few people who are registered opal valuers with the National Council of Jewellery Valuers of Australia (NCJV).