What is Opal?
What is Opal?
Opal is a form of silica, chemically similar to quartz, but containing water within the mineral structure. Precious opal generally contains 2-8% 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 (common opal with no colour play).
Colour play 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, much like the water particles do when creating a rainbow. 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 Australia occurs in rocks affected by weathering during the Tertiary Period (1.8-100 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 water table, possibly caused by changes in climate, carried silica-rich solutions downwards to form deposits and over time these hardened and turned into opal.
Value and Presentation
Attempts have been made to establish guidelines for determining a scale for opal prices but they have been largely unsuccessful because of the gem’s infinite variations. This is mostly due to certain criteria of opal being graded as more important by various stakeholders in the opal community. Black Opal miners and traders for example tend to overlook vibrancy and saturation of colour and focus just on the gems base body tone, which determines if it is classed as a black opal. Opal, like other gemstones, fits into the 4 C’s of gem grading; Colour, Cut, Clarity and Carats, with one addition being Pattern.
For a deeper insight on how to grade and value opal, please see our Grading Australian Opal page here…
Solid Opal
Solid opal is a 100% natural opal that has been cut and polished only. The colour-play and body tone (base colour) are exactly as it was found in its natural state. Usually, solid opals are cut ‘en cabochon’ (domed, rounded) as this highlights the colour-play and is most suited to setting in jewellery.
Solid opal can be:
- Full Colour, which means it shows play of colour in the entire stone.
- Layered Colour, where the play of colour appears in a band or bar, usually on potch (colourless opal) such as the majority of black opal or on ironstone in the case of boulder opal.
These are the only two varieties of opal we recommend.
Opal can also be Matrix Colour where the opal is impregnated into host rock. Matrix opal and fairy opal, this is the least expensive and most often treated or altered to show its play of colour. Always look for SOLID opal as this is true opal and has not been altered in any way from its natural state other than shaping and polishing.
