Is Opal a Gemstone?

Here, we provide a concise answer to the frequently asked question: “Is opal a gemstone?” We also discuss the gemmological properties of precious opal, opalescence and correct opal nomenclature (how to describe and classify opals).

Opal is a hydrated form of amorphous silica (SiO2 .nH2 O). It exists in two basic forms defined by visual properties[2][3].

1. Precious Opal
 
Precious opal exhibits a “play of colour” that is produced by the diffraction of light due to a microstructure of orderly arranged silica spheres. Precious opals are gemstones valued for their beauty.
 

2. Common Opal

Common opal does not reveal a “play-of-colour” and is typified by a measure of micro-crystallinity. It is similar in structure to precious opal but are further distinguished by a disorderly arrangement of silica spheres.

Most common opals are called potch. Potch differs from other common opals in formation and structure. It is structurally similar to precious opal but has a disorderly arrangement of silica spheres, whereas other common opals exhibit a degree of micro-crystallinity.

It may seem strange to ask: “Is opal is a gemstone?” However, if you consult a jewellery and gemstone directory, you will notice merchants who describe themselves as being ‘opal dealers’ or ‘opal and gemstone dealers’. It is a stands tribute to the opal’s uniqueness. 

Yet, precious opals are gemstones – they are referred to as either being gemstones or opals of gemstone quality, and they are distinguished by their ‘play-of-colour’. Moreover, opal is Australia’s officially designated national gemstone[1].

A Fabulous Black Opal (00:14 minutes)

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Opalescence: Understanding Play-of-Colour

It wasn’t until the 1960’s that studies by Australian scientists at The Commonwealth Scientific and Industrial Research Organisation (CSIRO) revealed the true nature of the play-of-colour within a precious opal – its opalescence[3].

Using an electron microscope, scientists were able to establish that opals were made up of millions of small silica spheres, 000005–0.0004 mm in diameter.

In the case of precious opals, it’s the size of the silica spheres, their arrangement within an orderly network and the light source that determines the nature of a stone’s play-of-colour.

Under a bright light-source, a precious opal’s play-of-colour can be observed from three aspects:

  1. The play-of-colour revealed by moving the stone
  2. The play-of-colour revealed by moving the light source
  3. The play-of-colour revealed by altering the angle of observation
Idealised Molecular Structure of Precious Opal | Dpulitzer [CC BY-SA]

Opals: The New Nomenclature

Traditionally, an Australian opal was informally classified in number of ways, usually related to related to its state or district of origin and its physical characteristics. However, as the Australian opal became ubiquitous in markets across the globe, the need for a standardised system of opal classification became apparent.

In 1993, The Australian Gemstone Industry Council approached the Gemmological Association of Australia (GAA) to propose a new opal nomenclature that would be uniformly accepted[2]..

The GAA resolutions that codify and standardise opal nomenclature were informed by the definitions and rules for gemstones, diamonds and pearls outlined by The International Jewellery Confederation (CIBJO).

The Nomenclature Subcommittee of the GAA, in collaboration with the Australian Gemstone Industry Council, the Australian Gem Industry Association, the Lightning Ridge Miners Association and the Jewellers Association of Australia, established uniformed standards for the classification of opals.

The NSW Department of Planning, Industry and Environment (Division of Resources and Geoscience)[3] has outlined the guiding principles here: Opal Classification.

How the Australian Opal is Formed

Typically, the Australian opal fields are located in arid regions, and opals are usually discovered within weathered rocks.

The process of opalisation commenced when water from infrequent rains, bearing dissolved silica, permeated the cracks and crevices of ancient, outback bedrock. During prolonged dry periods, as the water content evaporated, the silica solidified and began to form opal veins.

Australian opals are mined in South Australia, New South Wales and Queensland.

Elsewhere in the world, opals are usually found in regions associated with volcanic activity, and they often have have a high water content – as much as twenty percent of an opal’s mass can be water trapped within silica. Opals formed under these conditions are typically less stable than Australian opals and more prone to cracking and crazing. Crazing results in a loss of colour and brilliance.

The Properties of Precious Opal

Opal is a hydrated form of Silica. Water can represent as much as 20% of an opal’s weight, but generally lies within a range of 6% -10%. Precious opal is classified as mineraloid – it has an amorphous crystal system.

Gemmological Data: Precious Opal

Family:Mineral: Hydrated Silica
Crystal System:Amorphous
Specific Gravity:2.15
Hardness (Mohs Scale):5.0 – 6.5
Refractive Index:1.450
Chemical Composition:SiO2.nH2O
Birefringence:None
Pleochroism:None
Colours:All
Lustre:Sub Vitreous – Waxy
Fracture:Conchoidal

References

  1. Australian National Gemstone | Department of the Prime Minister and Cabinet. Retrieved 3 January 2020, from https://www.pmc.gov.au/government/australian-national-symbols/australian-national-gemstone
  2. Smallwood, Anthony. (1997). A new Era for Opal Nomenclature. Australian Gemmologist (1997) 19, 486-496.
  3. Opal classification – NSW Resources and Geoscience. Retrieved 10 January 2020, from https://resourcesandgeoscience.nsw.gov.au/miners-and-explorers/applications-and-approvals/opal-mining/about-opal/opal-classification
  4. Cram, L. (1998). A Journey with Colour (Volumes 1 -4). Lightning Ridge, N.S.W.: L. Cram.
  5. Cody, A. (1991). Australian Precious Opal. Melbourne: Andrew Cody Pty Ltd.