Formation of opal may include: | opal combining with other mineralsopal formed in:sedimentary depositvolcanic deposit (igneous)erratic bouldersreplacement cavities (nobbies, glauberite)seamsvolcanic and erosion sedimentsopal replacing:fossilswood. |
Sources of opal worldwide may include: | AustraliaBrazilHawaiiIndonesiaMexicoSlovakiaUSA. |
Locations of opal fields may include: | New South Wales – Lightning Ridge and White CliffsQueensland – various places including Winton, Quilpie, Jundah and YowahSouth Australia – Coober Pedy, Mintabie and Andamooka. |
Base and background colours may include: | blackboulder – ironstonedarklight. |
Predominant colours and patterns may include: | colours, such as:bluegreenorangeredvioletyellowpatterns, such as:broadChinese writingflagstoneharlequinpeacock tailpin firerolling flashsheenstraw. |
Other factors that influence an opal’s value may include: | availability of supplyeconomic climatemining restrictionsoriginpolitical situationrarityweightwhether the sale is wholesale or retail. |
Principles of valuing may include: | body colourbrillianceinclusion and imperfectionspatternplay of colourpredominant colourquality of cut (shape/proportions)shape, size and weightthickness of colour bartype. |
Gemmological characteristics may include: | chemical compositioncolourfracturehardnessphosphorescencerefractive indexspecific gravity. |
Gemmological instruments may include: | microscoperefractometerS.G. scalesstandard 10X loupeultraviolet lamps. |
Natural opal is classified as: | common opal and potchprecious opal. |
Substances used to simulate opal and synthetic opal-like substances may include: | simulants – Gilson opalsynthetics – for example plastics and slocum stone. |
Opals of various types may include: | composite natural opal – doublets, triplets, mosaic and chip opalsnatural opal type 1 – opal in one piece in its natural state apart from cutting and polishingnatural opal type 2 – opal naturally attached to the host rock – boulder opalnatural opal type 3 – opal intimately diffused as infilling in pores or between the grains of the host rock – matrix opalvarieties of opal: (N1 to N9 represents gradation from black to white in approximately 10% intervals)black – opal which shows a play of colour within or on a black or very dark body tone (N1, N2, N3, N4)dark – opal which shows a play of colour within or on a dark body tone (N5, N6)light – opal which shows a play of colour within or on a light body tone (N7, N8, N9). |
Opal treatments: | may include treatments applied to improve appearance, structure or durabilityany treatment other than cutting and polishing must be disclosed. |