The Opal can be called the most mysterious of all gemstones. Every opal is different and individual, one of the most spectacular precious gemstone. Nature has produced a fascinating display of all colours of the spectrum by arranging amorphous silica spheres of similar size in regular layers in an infinite variety of shades, patterns and brilliance, exposed as unique precious opal. Red is rarest and can only be produced when large sphere size is present (we are talking about sizes less than 1/1000mm).

What is Opal?

Opal = Si2.nH2O Non- Crystalline form of Hydrated Silica and contains 4% to 9% water.
Hardness: 5.0 -6.5 a mineral of medium hardness
Specific Gravity: Very low 1.9 to 2.3
Occurring as Veins and Nodules (nobbies)

Opal derives its name from the Roman word, "Opalus," meaning 'to see a change (of colour)'. The early Greeks thought opals gave their owners the powers of foresight and prophecy. Romans adored the opal as a token of hope and purity. Eastern people regarded it as sacred and Arabs believed it fell from heaven. Historically, opal has long been associated with royalty. A beautiful opal called 'Orphanus' was set in the crown of the Holy Roman Emperor and was said to guard the regal honour. Opals are also set in the crown jewels of France. Napoleon gave Josephine a beautiful opal with brilliant red flashes called 'The burning of Troy', regarding her to be his 'Helen'. Shakespeare compared the play of colour to shifting inconstancy of the mind.

How should one choose an Opal?

Each opal is very unique which is described as having character and therefore choice is a matter of your personal taste. Explore the full range of shapes, colours and varieties, some of which are seldom seen outside of Australia, and choose the one that appeals to your own personality and style. Opal is birthstone for June & October and presented for the 14th & 18th wedding anniversary. 

Gemstone Healing Guide:  http://www.freespiritemporium.com/guide.html 

Opal helps with stomach and intestine problems, recall of past lives, aids inner beauty, faithfulness and eyesight. The value of Opal is based on the following criteria: brilliance or lustre (brightness of colour), rarity of the colour or spectral colour range, type of pattern, any unique colour pattern, face-up Display (not only showing its beauty from acute angles), character, cut, shape, size, inclusions and imperfections. In these categories opal is priced per carat weight (0.2g). Boulder Opal is occurring in thin veins in sedimentary ironstone boulders found mainly in the arid regions of western Queensland and is unique to Australia. It is naturally bonded to its host rock, dense brown ironstone. Boulder opal may be light, dark or black. By modern designers boulder opals are used cut as well as shaped like its natural features are showing. In the last twenty years this type of opal has become extremely popular as it can display the same darkness and brilliance as a high quality black opal. The Yowah Nut received its name from the place where it is found and from its shape. It is a small boulder containing most diverse kernels of opal patterns and is perhaps the most unusual variety of boulder opal. Cut and polished, they inspire many artists to create extraordinary jewellery. Where the Opal is mixed through the ironstone it is called Matrix Opal. Opal can also be found as a pseudomorph replacing fossil shells, bones, or wood. The bodymass (potch) consists of random sphere sizes in irregular layers, and is opaque, usually white, grey, bluish, brown or black. Precious Opal often forms as a layer on potch. Black Opal has a natural dark or grey Potch base or dark body colour against which the play of colour shows up brilliantly. To find a matching pair can be very difficult. Almost all of the world's supply of Black Opal is Australian, mainly from Lightning Ridge NSW. Quality black opals can fetch prices equivalent to a good diamond on a per carat basis.

Lately dark Opals were found in Mintabie S.A., similar to black opal. Light Opal mainly comes from Coober Pedy, Mintabie and Andamooka in South Australia. The descriptive names "white", "milky", "grey", and "semi-black" Opal refer to the natural potch base or body colour of the Opal.

Crystal Opal is translucent with no opaque potch base and found at all opal fields. Some Crystal Opals show no colour unless put on a black background. These stones, when used for jewellery are usually set enclosed with background e.g. blackened silver or cement. This does not detract from the price. 

Jelly Opal is very transparent.

The back of Opals is not always polished and can have  inclusions. This does not effect the price.

Sometimes a cutter polishes the back to give the option of setting the stone either way.

Opal Nomenclature and Classification

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 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 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.Both doublets and triplets imitate black opals, but are a fraction of the cost.
• 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.References

Australian Opal and Gem Industry Association PTY LTD

Care for your Opal

• Protect the stone from hard hits of other Jewellery items or heavy work e.g. bricklaying – store separately. Avoid extreme sudden temperature changes and very low humidity (desert or bank vaults).
• Never use harsh chemicals, bleach or silver-dipping solutions to clean opal jewellery. Careful with ultrasonic cleaners and steamers! A mild soapy lukewarm water solution and a very soft brush may be used for jewellery set with solid opals, but never immerse doublets or triplets in water as the glue may deteriorate or get soaked, which could make them dull. The safest cleaning method is to use a soft damp cloth followed by a jeweller's polishing cloth for the metal.
• Finished opals need not be oiled or soaked in water periodically. Some people believe that oil protects the colour. In fact oil hides cracks in a stone, but discolours and dulls the stone over time.
• If an opal does become scratched and opaque over time, the surface can usually be re-polished by a qualified jeweller or gem cutter.