What Is Opal? 

What Is Opal? 

Opals (courtesy of Gryffin Opal)
Opals from Coober Pedy, South Australia (Courtesy of Gryffin Opal)

Opal is a form of silica, chemically similar to quartz, but containing water within the mineral structure. Precious opal generally contains 6-10% water and consists of small silica spheres arranged in a regular pattern. Opal occurs in many varieties, two of which are precious opal and potch.

Colour in precious opal is caused by the regular array of silica spheres and voids diffracting white light, and breaking it up into the colours of the spectrum. The diameter and spacing of the spheres controls the colour range of an opal. Small spheres (approx. 150-200 nm; I nm = 1 0’9 m) produce opal of blue colour only, whereas larger spheres (350 nm) produce red colour. Opal with red colour can display the entire spectrum. Opal colours also depend on the angle of light incidence and can change or disappear when the gem is rotated.

Geology of Opal

All precious opal in South Australia occurs in rocks affected by weathering during the Tertiary Period (1.8-70 million years ago). The weathering process broke down minerals of the country rock to produce kaolin (a clay) and soluble silica. It also created cavities in the rock by dissolving soluble minerals and fossil shells. These cavities, together with faults and fractures in the rock, provided pathways for underground water containing the soluble silica. Periodic lowering of the watertable, possibly caused by changes in climate, carried silica-rich solutions downwards to deposit opal in the rock cavities.

Value and Presentation

Attempts have been made to establish guidelines for determining opal prices but they have been largely unsuccessful because of the gem’s infinite variation in colour pattern.

The main factors influencing the price paid for opal are:

  • Background colour – black opal (a gem with a dak background) is more valuable than clear opal (crystal opal) which in turn is generally more valuable than white or milky opal.
  • Dominant fire colour – red-fire opal is generally more valuable than a predominantly green opal, which in turn is more valuable than a stone showing only blue colour.
  • Colour pattern – Harlequin opal, where the colour occurs in regular patterns such as a checkerboard or diamonds, is the rarest and thus most valuable. Other notable patterns are Flagstone, Chaff, Pinfire, Floral, Chinese Writing, Galaxy, Palette and Rolling Flash. You can have multiple patterns in one stone. In valuation, brightness and quality of colourplay is more important than pattern.

There is a large difference between the value of uncut opal and that of a cut and polished stone. Solid opals are most often cut cabochon (rounded) as faceting does not enhance most types of Australian opal, with the exception of Australian Fire Opal varieties. Under the Trade Standards Act, all opal sold in Australia must be clearly labelled to show the type of opal and how it is presented.

Solid (cabochon)

A solid opal is a 100% natural opal that has been cut and polished only, the colourplay and body tone (base colour) are exactly as it was found in its natural state. Usually, solid opals are cut ‘en cabochon’ (rounded) as this highlights the colourplay and is most suited to setting in jewellery.

Manufactured & Fake Opal

Doublet

This is considered Manufactured Opal. A thin veneer of opal may show enhanced colour with a dark backing. This can be achieved by cementing either black or grey silica material or a thin slice of common opal to the back of the opal with epoxy resin.

We do not sell manufactured opals. All Gryffin Opals are 100% natural solid Australian Opals.

Triplet

This is considered Manufactured Opal. A slice of quartz may be used to cap the thin opal veneer to protect it from abrasion. This produces a three-tiered gemstone known as a triplet, which can often display brilliant colours. It is a cheaper method of presentation and can enhance the appearance of the opal.

We do not sell manufactured opals. All Gryffin Opals are 100% natural solid Australian Opals.

Treated Opal

Many opals from outside Australia are treated to enhance their colour. Mostly used on unstable opal varieties such as Hydrophane, (often from Ethiopia) and Indonesian opal. The treatments often fade in a short period and make the opal even more unstable. With the exception of Australian Matrix opal, we advise not to invest in treated opal at all and consider it as fake.

Imitation Opal

There is a large variety of imitation and synthetic opal on the market. This is not opal and mostly Plastic or Resin. Always check the opal merchant offers a Certificate of Authenticity that your stone is a 100% natural Australian Opal. It is an offence under Australian law to sell fake opal as natural opal, so purchasing from a trusted Australian supplier who offers certificates and guarantees will give you peace of mind in your opal investment.

We do not sell imitation opals. All Gryffin Opals are 100% natural solid Australian Opals.