Opal characteristics Opal classification Types of opal Varieties of opal Body tone Transparency Play-of-colour Brightness Pattern Shape and face Origin


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Opal Lexicon

Types of Natural Opal

The Australian Gemstone Industry Council, the Australian Gem Industry Association (AGIA), the Gemmological Association of Australia, the Lightning Ridge Miners Association and the Jewellers Association of Australia have produced a system for the nomenclature and classification of opal by the subdivision in Opal Type and Variety which is determined by Body tone and Transparency. A thorough explanation of these terms can be found here and on the following pages.

OPAL TYPE 1

This type is commonly known as SOLID opal. A single piece of opal with a more or less homogenous chemical composition. However some parts of the piece may be precious opal and others may be potch and it still may contain minor remains and inclusions of sandstone and other non-opal materials.

OPAL TYPE 2

This type is commonly known as BOULDER opal. A layer of opal that is still naturally attached to the host rock in which it was formed (e.g. ironstone or any other rock material). The reason for this is that the layer of opal is usually too thin to be cut without retaining a backing of host rock.

OPAL TYPE 3

This type is commonly known as MATRIX opal. The material consists of a conglomerate of minute grains of opal which are diffused as fillings of pores or holes or between grains of the host rock in which they were naturally formed e.g. Andamooka Matrix Opal.
BOULDER MATRIX is a special form of matrix opal with a different structure compared to the granular build-up of Andamooka Matrix Opal. It usually shows many fine veins and small patches of opal surrounded by host rock.

 

Type 1 Type 2 Type 3 Type 3
Solid Opal Boulder Opal Treated Andamooka Matrix Opal Boulder Matrix Opal

Solid Opal

Boulder Opal

Andamooka Matrix Opal
(treated with sugar/acid)
Boulder Matrix Opal

 

Note: These official three types apply only to natural opals. Other kinds or types of opal-like materials as opal imitations, composites (e.g. doublets and triplets) and artificial or "synthetic" opals (e.g. Gilson and Kyocera Opals which can also be considered opal imitations since they have a slightly different composition and lack the water content compared with natural precious opal) have no relevance in this kind of classification.
 

Opal Doublet cross-section Opal Doublet

Construction of a Doublet

Opal Doublet

 

Opal Triplet cross-section Opal Triplet
Construction of a Triplet Opal Triplet

 

Synthetic Gilson Black Opal Lizard skin structure in Gilson Opal

Gilson Black Opal

Chicken wire or lizard skin
structure in Gilson Opal

 


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