Classification 

The classification of opal has always presented difficulties and many different names have been given to opals. Broadly speaking opal types have often been named by:-

  • their value (precious opal, common opal), 

  • their association with other materials (boulder opal or matrix opal), 

  • their mode of formation (Nobby opal, pipe opal or seam opal), 

  • their geological occurrence (sedimentary opal or volcanic opal), 

  • their mineralogy (Opal-A, Opal-CT, Opal-C)

  • their body colour or tone (Black opal, White opal or Fire opal), 

  • their replacement of other things (wood opal or shell opal), 

  • the quality of their Play-of-Colour (brightness, colours, consistency

  • their type of pattern (harlequin opal, pinfire opal) 

  • by geographical location, (Mexican opal, Lightning Ridge opal). 

  • their other phenomenal observable details, (moss opal, cats eye opal)

  • their fossil or paleontological characteristics

In 1999 an opal nomenclature was introduced in collaboration with The Gemmological Association of Australia (GAA), The Lightning Ridge Miners Association (LRMA), The Jewellers Association of Australia (JAA) and the National Opal Miners Association (NOMA). The nomenclature was tabled at CIBJO and accepted as a suitable nomenclature for describing opal.

Recent (2014) discussions within the International Coloured stone Association (ICA) subcommittee GILC have agreed to add a fourth opal type to the nomenclature with the advent of opal discoveries in Ethiopia.

Much of the Ethiopian material is of the mineralogical variety of opal known as “Hydrophane”. It is so called because of its porosity and ability to absorb water. 

As this opal is essentially “all opal” and technically falls into the opal “Type 1” category (as published in the Australian Gemmologist). It is unclear as to how the nomenclature will change. It may well be that Type 1 will be subdivided into Type 1A and Type 1B. The other option is to create Precious opal type 4 as Precious Hydrophane Opal. This is yet to be determined.

Opal Types

Natural Opal (Type 1) or Type 1A

This is opal which is presented in one piece in its natural state other than cutting or polishing and is of substantially homogenous chemical composition.(eg: black, dark, light and orange body or “Hue” coloured “Mexican” type  opal.)

Natural Hydrophane opal (Type 1B) or Type 4

This is opal which is presented in one piece in its natural state other than cutting or polishing and is of substantially homogenous chemical composition

Natural Boulder opal (Type 2)

This is opal presented in one piece where the opal is naturally attached to the host rock in which it was formed where the host rock is of different chemical composition, and is called boulder opal.(eg: “Queensland” type boulder opal, or “Angel stone” quartzite boulder opal from Andamooka, South Australia.)

Natural Matrix opal (Type 3)

This is opal presented in one piece where the opal is intimately diffused as infillings of pores or holes or between grains of the host rock in which it was formed and is called matrix opal.(eg: “Andamooka” type matrix-“Queensland” type boulder matrix opal).

The body “Colour” or body “Tone” of opal

The term “Body Colour” refers to the relative lightness or darkness of “Colour” as used in modern colour science terminology. The Scientific term and that preferred by the American appraisers is “Body Tone”

The body tone of an opal is different to the play-of-colours displayed in precious opals. Natural opal varieties are described in five basic ways. Black and Dark opal, Light opal, Crystal opal (transparent) and “hue” or “body” coloured opal. (Refer to attached copy of the Opal Nomenclature).

Black opal

Is precious opal that shows a play-of-colour within or on a black or very dark body or background colour (tone) when viewed face up. 

Dark opal

Is precious opal that shows a play-of-colour within or on a medium dark grey to a medium/light grey body or background colour (tone) when viewed face up. Dark opal is a separate category from Black Opal. The distinction has been made by comparison of present day marketing trends. 

Light opal

Is precious opal that shows a play-of-colour within or on a predominantly very light greyish, or white body or background colour (tone) when viewed face up. If the opal is very white and “milky” it is termed as white opal.

Crystal opal

Is precious opal that shows a play-of-colour within a transparent to translucent or “clear” background. It can be termed as black, dark or light crystal opal. Note: the term jelly opal is no longer used as it now represents a “poor” quality of crystal opal. 

Coloured Body opals

“Hue” or “body” coloured opals such as those that are red, yellow and green are opals that have a distinctly “coloured” body colour when viewed face up (Like Mexican fire opal). Some of these opals show a play-of-colour and some do not. The can also be described like other gemstones as “Transparent” or “Opaque”, and light or dark.