Opal is a gemstone consisting of hydrated amorphous silica with the chemical formula SiO2.nH2O. There are two basic forms of opal described by visual appearance.
Precious Opal - is opal which exhibits the phenomenon known as play-of-colour, produced by the diffraction of white light through a micro-structure of orderly arrayed silica spheres to produce changing spectral hues.
Common Opal and Potch - is opal which does not exhibit a play-of-colour. The distinction between common opal and potch is based on formation and structure. Potch is structurally similar to precious opal but has a disorderly arrangement of silica spheres. Common opal shows some degree of micro crystallinity.
Natural opal is opal which has not been treated or enhanced in any way other than by cutting and polishing. There are three types of natural opal, with varieties described by the two characteristics of body tone and transparency.
Natural Opal Type 1 - is opal presented in one piece in its natural state apart from cutting or polishing and is of substantially homogenous chemical composition.
Natural Opal Type 2 - is opal presented in one piece where the opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition. This opal is commonly known as boulder opal.
Natural Opal Type 3 - 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. This opal is commonly known as matrix opal.
The variety of natural opal is determined by the two characteristics of body tone and transparency.
The body tone of an opal is different to the play-of-colour displayed in precious opal. There are three varieties of natural opal based on body tone. Body tone refers to the relative darkness or lightness of the opal when ignoring the play-of-colour.
Black Opal - is the family of opal which shows a play-of-colour within or on a black body tone by reference to the AGIA Body Tone Chart N1, N2, N3 and N4 when viewed face up.
Dark Opal - is the family of opal which shows a play-of-colour within or on a dark body tone by reference to the AGIA Body Tone Chart N5, N6 when viewed face up.
Light Opal - is the family of opal which shows a play-of-colour within or on a light body tone by reference to the AGIA Body Tone chart N7, N8 or N9 when viewed face up. The N9 category is referred to as white opal.
Opal with a distinct coloured body (such as yellow, orange, red or brown) should be classified as black, dark or light opal by reference to the AGIA Body Tone Chart with a notation stating its colour hue.
Opal shows all forms of diaphaneity and ranges from transparent to opaque. Natural precious opal which is transparent to semi-transparent is known as crystal opal. Crystal opal can have either a black, dark or light body colour tone. The term "crystal" in this context refers to appearance not a crystalline structure.
Black opal, precious opal with black body color. Also used for black potch covered with thin layer of crystal opal that lets the black under layer show through.
A Black Opal is so named because of its natural dark base colour which can be black, dark blue or dark gray. The surface colours can range from a variety of blues and greens to all the brilliant colours of the rainbow. It is found in two centres of Australia---Lightning Ridge in New South Wales and Mintabie in South Australia. The Lightning Ridge Opal is the better of the two, due to brilliance and intensity of colour. The Lightning Ridge Black Opal is the most expensive of all the opal types, on a dollars per carat basis, due to its absolute rarity and the demand from collecters around the world.
Semiblack, or grey opal, precious opal with dark body color.
White opal, precious opal with white or very light body color. The White Opal has a whitish base colour all over the gemstone. The brilliant colours emanate from this white base. One of the differences between White Opal and Black and Boulder Opal is the colour play generally extends from surface to base, or from skin to skin, whereas the other two types have a naturally occuring single base colour, differing from its surface colours.With a light texture base, this material can appear from a superior crystal gel form emerging with interplay of different firy bright colours down to a purely white milky solid. It is usually drawn from the mines in Coober Pedy, Andamooka in South Australia.
Crystal opal, transparent to semitransparent, colorless body with play of color.
Doublets and Triplets make use of opal that is too thin to use as a solid gemstone. A doublet is a thin layer of precious opal glued to a black base. A triplet adds a transparent, quartz cap. ... Purists prefer the base material to be common opal. However many black materials are used, including old phonograph records
Doublet Opal. Same as a triplet, but no quartz top. The Opal slice is usually rounded a little for a cabachon effect. It is made by gluing slices of precious opal to a common opal backing with blackened cement, usually an epoxy resin. The dark backing enhances the colours of the opal. Boulder Opal Doublet comprises of a layer of white opal on the top and attached by another layer of iron-backing stone from Boulder Opal underneath.
Triplet / Triplex. It is a three part piece consisting of the base (either vitrolite, plastic or obsidian), a centre slice of White Opal which has been painted black on the underside to give a Black Opal "look alike" appearance and has a top of Quartz as a protection for the thin layer of Opal (which is usually sliced to only 1000th of an inch).
Black crystal opal, transparent to semitransparent opal, with dark body color and play of color.
Fire opal, translucent to transparent, with yellow, orange, or red body color. May or may not have play of color. Also called Mexican opal or Sun opal.
Boulder opal, a thin seam of precious opal on ironstone matrix. Since this is a natural occurrence, its value is higher than that of a man made doublet. The ironstone is very dark, which makes the fire stand out and gives a close resemblance to black opal.
Boulder Opal. Found only in Queensland, this rare Opal was formed from silicon diozide and other minerals finding their way into the fissures of ironstone. The cutter must always take a little of this natural ironstone when extracting a gemstone and this gives the dark overall colour which does not detract from the Boulder Opal. When little or no ironstone appears in the face of Boulder Opal, the price of Boulder equals that of very fine Black Opals. The natural occurrence of Boulder allows for particularly interesting shapes which in the hands of a creative jeweller makes for stunning creations.
Matrix, There are two types of Matrix, one is known as Andamooka Matrix which is sugar and acid treated, Opal bearing rock, and the other is Queensland Boulder Matrix. Queensland Boulder Matrix is natural, but is not included in this collection except for a few bead items. It is simply naturally-occuring Opal and ironstone in a pepper and salt pattern.
Matrix opal consists of thin seams or spots of precious opal in matrix. The matrix is usually dark ironstone, but occasionally a light sand stone. Also called Mass opal. Opal also shows one of the best spectral displays of any gemstone, hence its value. It is made up of layers of precipitated silica spheres in a jelly-like water mass, and the ordering of the spheres sometimes produce a diffraction grating, that creates a play of rainbow sparkling light from within the stone.Matrix opal consists of thin seams or spots of precious opal in matrix. The matrix is usually dark ironstone, but occasionally a light sand stone. Also called Mass opal. Opal also shows one of the best spectral displays of any gemstone, hence its value. It is made up of layers of precipitated silica spheres in a jelly-like water mass, and the ordering of the spheres sometimes produce a diffraction grating, that creates a play of rainbow sparkling light from within the stone.