Opal Nomenclature

The terminology and nomenclature describing opals has been widely discussed and debated by members of the industry and gemmologists alike. How to best describe opal has become a contentious and frustrating issue. Through a growing international and local awareness of opal as a major resource, the desire for a standardised set of terminology related to opals has increased dramatically. It has therefore become necessary to agree on some well based concepts of how a unique material, such as opal, should be described.

The information below has been reproduced from The Opal Association.

Opal Classification

An amorphous silica mineraloid with the chemical formula SiO2 .nH20. Opal comes in many unique varieties based on the location it was sourced from. There are two basic types of opal that is described by the visual appearance, precious opal and common opal. Precious opal exhibits the phenomenon known as play-of-colour. Common opal and potch, are opals that do not exhibit this phenomenon. The majority of opal is mined in Australia where over 96% of the worlds supply is produced. Notable mines in Australia include Coober Pedy, Lightning Ridge, Lambina, Mintabie, Andamooka, Yowah, Koroit, Jundah, Winton and Quilpie. Other countries that produce both precious and common opal include; Brazil, Czech Republic, Ethiopia, Guatemala, Honduras, Hungary, Indonesia, Mexico, Nicaragua, Peru, Slovakia and the U.S.A.

Precious Opal

Precious opal is determined by opal that presents a “play of colour” along the surface of the stone. This type of opal flashes with iridescent colours when viewed from different angles, when the light source is moved or the stone itself is moved.

Common Opal

Common Opal is found throughout the world and does not possess a “play of colour” like precious opal. It can be vividly colourful and can vary from opaque to transparent.

Types of Natural Opal

Natural opal has not been treated or enhanced in any other way than cutting and polishing. Three of the types of natural opal are described by the two characteristics of body tone and transparency.

Whole

Presented as a single homogenous piece in its natural state apart from cutting or polishing.

Attached

Presented fused with a host rock where there is a different chemical composition between the two parts.

Diffused/Infilled

Presented where the opal is intimately diffused throughout a host material infilling pores, holes or between grains.

Varieties of Natural Opal

Transparency
Transparent OpalOpal shows all forms of diaphaneity that range from transparent to opaque.
Precious Opal that is semi-transparent to transparent is known as crystal opal,
it can have either a black, dark or light body tone.

Body Tone

The body tone of an opal differs from the play-of-colour displayed across precious opal. Referring to the relative darkness of the opal, ignoring its play-of-colour. This is assessed on a Scale of Body Tone.

Black Opal: N1 - N4, Dark Opal: N5 - N6, Light Opal: N7-N8, White Opal: N9

Black Opal

Black Opal: N1 - N4

Black Opal is mainly found at Lightning Ridge Australia. It is the most valued of all opals with a distinct block body tone or base that distinguishes it from all other types of precious opal.

Dark Opal

Dark Opal: N5 - N6

Dark Opal is classified as N5 or N6 using the scale for body tone. Although not as dark as black opal this opal still shines vividly. Dark opal is relatively common

Light Opal

Light Opal: N7 - N8

Light opal is classified as N7 or N8. Light Opal is very common and has been mined from coober pedy since it was first discovered there.

White Opal

White Opal: N9

White opal can be classified as opal of N9. Along with light opal, white opal is the most common form of precious opal with a milky white body tone.

Opal Treatments

Opal can be treated in a variety of ways. These include colour and/or tone enhancement, heating, painting, dying, resins and waxes, oiling or application of chemicals. This is usually to enhance the colour or increase the gems durability.

Composite Natural Opal

Composite natural opal consists of opal laminates, manually cemented or attached to another material. There are three main forms of composite opal: doublets, triplets and synthetic opals.

ACKNOWLEDGMENTS

REFERENCES

  • Opal Association
  • Altman, J.D. (1979) Suggestions for nomenclature of opals. Australian Gemmologist. 13(12), 383-385.
  • Clayton, N.A. (1975) Classification and nomenclature of precious opal.Australian Gemmologist. 12(5), 152-154.
  • Darragh, P.J., Gaskin, A.J., and Sanders J.V. (1976) Scientific American, 234(4), 84-94.
  • Editorial Committee of The Australian Gemmologist (1971)
  • Some thoughts on opal. Australian Gemmologist. 11(2), 24-26.
  • Geological Society of America (1991) Rock-color chart. GSA: Boulder, Colorado.
  • Haugen, S.O. (1987) A system for evaluation of opal. Australian Gemmologist. 16(6), 213-215.
  • Herbert, P., Hearnes J., and Males, P.A. (1972) Opal nomenclature discussion.Australian Gemmologist 11(5), 23-24.
  • Jones, J.B. and Segnit, E.R (1971) The nature of opal – Opal nomenclature and constituent phases. Geological Society of Australia. 8(1), 57-68.
  • Kalokerinos, A. (1971) Black opal nomenclature. Australian Gemmologist 11(4), 16.
  • Munsell, A.H. (1961) A colour notation. Munsell Colour Co: Boston, USA.
  • New South Wales Opal Research Group (1971) The classification of black opal. Australian Gemmologist. 11(2), 24.
  • Sanders, J.V. (1983) A proposal for the classification of opal. Australian Gemmologist. 15(3), 75-78.
  • Sherman, G. (1983) Certification of opal. Australian gemmologist. 15(3), 71-74
  • Wyszecki, J. and Stiles, R. (1982) Colour Science.Wiley: New York.
  • Copies of the Rock-color Chart may be purchased either from The Geological Society of America, P.O. Box 9140. Boulder Colorado 80301 (for US$26.00, plus postage), or from
  • Prospectors, P.O. Box 339, Seven Hills NSW 2147 (for $A48.00).