Opal Technical Info
Chemical formula Si02.nH20 and the name deriving from the Ancient word ”opalus”, meaning to see a play of colours, opal's unique precious gemstone consists of Hydrated amorphous Silica. The unique ‘play of colours’, characteristics of Opal, are caused by a myriad of minute silica spheres cemented together by silica gel.
RI: 1.450
Birefringence: None
Optic Character: Single Refractive
Specific Gravity: 2.15
Hardness: 6 average
Transparency: Opaque to TP
Special Identifying Properties and Tests: Play of color, low refractive index, SG, magnification should be diagnostic.
Synthetics: Gilson synthetic is easily identified by its snake skin appearance. I have slides of this material that I will be adding. Glass imitations are easily identified by their coloration and varying RI.
Imitations: Many
Australian opals are found in irregular nodules or “nobbies” or in steams, the ‘Australian Black ‘Opal gets its name from the background colour which maybe a dark grey known as a semi-black to an intense black background which is the most desirable. Visual colours or ‘face up display’ which is term to indicate colour directly to the viewer, range through blue, green, orange and red. The finest and rarest of gems occurs when ”red on black” combines to show a full array of spectral colours.
Opal is a form of silica, chemically similar to quartz, but containing water within the mineral structure. Precious opal consists of small silica spheres, of uniform size and arranged in a regular pattern.The colour in precious opal is caused by the regular array of silica spheres diffracting white light and breaking it up into the colours of the spectrum. The play of colour in opal depends on the angle of incidence of the light and can change or disappear when the gem is rotated. The size and spacing of the spheres also controls the colour range of opal. In opals showing reds flashes, the spheres are larger than those showing only violet or green flashes.In potch opal and common opal the silica spheres may be absent or too small or irregularly arranged to produce colour.
It took the development of the electron microscope to work this out. Precious opal is made up of tiny uniform spheres of transparent hard silica, which fit together in an orderly three dimensional frame, sitting in a "bath" of silica solution. It is the orderliness of the spheres that separates precious opal from common opal.
Light passes through the transparent spheres in a direct line, but when it hits the 'bath' of silica, it is bent and deflected at different angles, thus producing a rainbow effect.
Hydrated silica material, made of submicroscopic silica spheres held together by more silica and water. It is a soft stone, easily altered in appearance by changes in heat and pressure. This mineral contains varying amounts of water within it that determine the appearance of the gemstone. When water evaporates out of an opal, the stone appears slightly smaller and the stress of the evaporation creates cracks on it.
Volcanic rocks, within cavities and cracks. In sedimentary volcanic ash rock, percolating water in the ground dissolves silica that eventually precipitates to form the opal, sometimes becoming the replacement material for fossils -- shells, bones, wood -- whose original material had dissolved away.
Opals play of colors many stones flash the colors of the rainbow when moved, due to the interference of light on small cracks and other internal structural differences. Opals also have characteristic colors due to impurities within the stone. The milky or pearly appearance of some opals are due to inclusions of tiny gas bubbles. Yellows and reds betray the presence of iron oxides. The spectacular black opals that sometimes flash green, blue and red get their color from magnesium oxides and organic carbon within the stone. Perhaps the most valuable opal pattern is the "harlequin," large angular patches of red, yellow and green resembling the checks on a clown's costume.
Deflection & Diffraction
Depending on the size of the spheres, varying colours of the spectrum are diffracted. So it is a combination of deflection (bending) and diffraction (breaking up) of light rays that creates the colour in opal. If you move the stone, light hits the spheres from different angles and bring about a change in colour. The name opal actually means "to see a change in colour." The way in which colours change within a particular stone as it is rotated and tilted is called the stone's play of colour.
How colour is defined
The size of the spheres has a bearing on the colour produced. Smaller spheres bring out the blues, from one end of the spectrum. Larger spheres produce the reds from the other end. The more uniform the spheres are placed, the more intense, brilliant and defined the colour will be.
OPAL NOMENCLATURE AND CLASSIFICATION Australian Gemstones: Opal INTRODUCTION Opal is Australia's National Gemstone. Australia produces 95% of the world's natural precious opal supply. This nomenclature encompasses all types and varieties of opal to provide a standardisation of terminology but does not establish any valuation methodology. The Australian Gemstone Industry Council Inc., in collaboration with the Australian Gem Industry Association Ltd., the Gemmological Association of Australia Ltd., the Lightning Ridge Miners Association Ltd. And the Jewellers Association of Australia Ltd., has produced the following nomenclature for the classification of opal. OPAL CLASSIFICATION 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. TYPES OF NATURAL OPAL 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. VARIETIES OF NATURAL OPAL The variety of natural opal is determined by the two characteristics of body tone and transparency. Body Tone 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. Transparency 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. OPAL TREATMENTS Opal can be subjected to various types of treatment. Present CIBJO guidelines state that any method of treatment other than standard cutting and polishing must be disclosed and the process used specified on all invoices, advertising and commercial documents. Types of treatments include colour enhancement, heating, painting, dying, resins and waxes, oiling or any application of chemicals. Opal is treated to change its natural appearance, structure or durability. Opal is colour enhanced in opal inlay jewellery where usually a thin solid crystal opal has black paint or glue applied or set above black painted jewellery. COMPOSITE NATURAL OPAL Composite natural opal consists of natural opal laminates, manually cemented or attached to another material. The opal component is natural opal. There are three main forms of composite opal: Doublet Opals - are a composition of two pieces where a slice of natural opal is cemented to a dark base material. Triplet Opals - are a composition of three pieces where a thin slice of natural opal is cemented to a dark base material and a transparent top layer, usually of quartz or glass. Mosaic and Chip Opals - are a composition of small flat or irregularly shaped pieces of natural opal cemented as a mosaic tile on a dark base material or encompassed in a resin. SYNTHETIC OPAL Synthetic Opal is material which has essentially the same chemical composition and physical structure as natural opal but has been made by laboratory or industrial process. Synthetic composites exist as synthetic doublets, triplets or mosaics and must be disclosed as synthetic composites. IMITATION OPAL Imitation Opal is material which imitates the play-of-colour of natural opal, but does not have the same physical and chemical structure or gemmological constants as natural opal.
CLASSIFICATION REPORTS Classification reports for the following types of opal should include these details: Natural Opal 1. Type of opal 2. Variety of opal as Black opal, Dark opal or Light opal with a body classification from N1 (Black) to N9 (White) based on the AGIA Body Tone Chart. 3. Transparency as opaque, translucent or transparent. Note if it is crystal opal. 4. Weight and dimensions Treated Opal 1. Type of opal 2. Variety of opal as Black, Dark or Light opal 3. Transparency as opaque, translucent or transparent. Note if it is crystal opal. 4. Type of Treatment and process if known 5. Weight and dimensions Composite Opal 1. Type of composite as doublet, triplet, mosaic or chip opal 2. Treatment process, where relevant 3. Dimensions Synthetic and Imitation 1. Gemmological category including manufacturer (if known) 2. Description (Body Tone) 3. If composite, mention type as doublet, triplet, mosaic or chip 4. Weight and dimensions, only dimensions if composite Origin Any indication of the origin of opal by the use of geographical location should not be used unless it is qualified as an indication of the type of locality only as recommended by the International Confederation of Jewellery, Silverware, Diamonds, Pearls and Stones (CIBJO) such as Lightning Ridge type black opal. See also: What is an opal? AUSTRALIAN OPAL AND GEM INDUSTRY ASSOCIATION LIMITED A.C.N. 001 117 237 Suite 309, Grand United Building, 149 Castlereagh Street, Sydney 2000, Australia Phone: + 61 2 9267 1310 Fax: + 61 2 9267 1037
Buying opal in the rough
Opal uncut opal on the internet, buy samples first if possible. If some of the rough is faced it makes the opal rough parcel far less risk. If you're buying opal on the opal fields try to find clues in the rough parcel. Most rough is sold wet so you can see the colors as they will be when they are polished. This is standard practice. However it pays to wipe a few bits dry as this will show up any crazing or cracks often caused from contact with jackhammers or bulldozers etc ! Faced pieces will show up the general character of the whole parcel usually. Opal miners don't mind you taking a good close look at their product, but if you find something you don't like be discreet. Never knock or rubbish some ones opal..... They've probably worked really hard for it and they won't like unkind comments and you won't like being hung upside down by one leg over a deep mineshaft either!
Boulder Opal
Boulder opal gets its name from the fact that an ironstone backing is part of its natural formation, the colours can also be quite brilliant and dazzling and gem quality boulder opals also brings a high price almost matching the black opal prices. It would appear that there is a larger production and it is found over a much wider area (mainly Western Queensland ) than black opal and so the price of average grade material is somewhat lower.There is also boulder matrix opal which has tiny lines of colour running through the boulder host rock. This material is cheaper but attractive and popular in Germany as a semi-precious form of opal
Boulder Opal is found only in Queensland, Australia. It formed millions of years ago, when liquid silica filled voids or crevices in ironstone boulders. When cutting this type of opal, the natural rock is removed from one side of the opal, exposing the face of the stone. The natural stone is left on the back of the opal to give the opal its strength and bolder colors. Boulder Opal is a unique gem, found in every color of the rainbow. It is durable . Boulder Opal jewelry requires no special care, can be worn for all occasions.