Varieties of Opals
Opals come in three different categories solids, doublets and triplets.
Solid opals are the most valuable, cut from only the natural stone without any treatment or enhancement.
Within this category, there are three kinds of solid opal - Light and crystal opal, Boulder opal and Black opal.
Light and crystal opal ranges from a near opaque milky white to a transparent, translucent colour. The transparent varieties are commonly referred to as crystal and are generally the most beautiful form of light opal due to the amazing play of colour. They do not possess a true crystal structure the reference to crystal refers only to the translucence nature.
This type of opal is mined at Coober Pedy, Andamooka.
Boulder Opal can be dark or light and is found in thin veins of sedimentary ironstone boulders, the colour layers are naturally bonded to the dense brown ironstone base to such a degree that it is inseparable. This opal forms in horizontal and vertical cracks in the rock with better quality boulder found in horizontal seams. The quality of the boulder lessens as the thickness of the opal increases.
This type of opal is mined in Queensland.
Black opal is an opaque body ranging from a dark grey to black from which overlying spectral colours are enhanced. This occurs because the dark background of the opal absorbs the otherwise scattered white light developed in light varieties of opal which dilute and diffuse the colour play, thus enhancing brilliance. In most valuable black opal you can see a thin uppermost layer containing the colour play over a dark devoid of colour base(potch base).Black opal is generally the most valuable form of opal provided all other things are equal
This opal is mined at Lightning Ridge.
Doublets and Triplets Opals:
Doublets are a thin slice of translucent opal joined by an epoxy resin to a backing, usually either made from black potch or ironstone. The opal is cut thin so as the translucent opal will look darker, once placed on the dark backing, giving it the resemblance of a black or boulder opal. Their purpose is to provide an economical alternative to the gem quality black or boulder opals as their value is only a fraction of the cost of the real solid opal. They can be distinguished by looking at them from the cross section where a straight distinct line can depict the different layers.
Triplets are made by adding a clear protective quartz or crystal glass top to a doublet; the slices are usually thinner than a doublet so as to make them less expensive than doublets. The domed cap protects the opal and magnifies the colour and pattern improving the stone.
Both doublets and triplets should not be immersed in water or liquids as they may cause them the layers to separate with constant emersion
Often referred to as Gilson after one of the manufacturers, synthetic opal is manufactured in a laboratory, the colour patterns are usually amazingly bright and consistent, therefore making it hard to believe that it is real. The opal comes in the light and dark shades and the colour pattern is always the same which makes it different to real opal whose colour pattern tends to be more random. Solid synthetic opal can be easily detected when looking at a cross section of the opal, because the colour and pattern follow the stone in a vertical or horizontal fashion throughout the entire stone.
Andamooka matrix can be treated in a bath of sulphuric acid and sugar, boiled over a slow heat over 24 hours to enhance the matrix (darken the material)
Quality and Valuation:
There are a number of factors to take into account
Intensity or brilliance:
Of the play of colour is an important factor to take into account in a stone. A better stone will be clear and crisp in colour the shades of colour will be distinct and definite, intense without being cloudy Colour Background a darker based opal will generally be more valuable than the equivalent lighter based opal (all things being equal) with an intensely black background opal being the most desirable and expensive
Choosing an opal can be a subjective choice dependant on colour preference however when it comes to valuing a stone to sell, certain colours are considered more valuable due to scarcity factors.
Blue is a desirable and attractive colour but are less valuable because of their relative abundance , blue can vary from an cloudy light blue to an intense electric blue
Green tends to be more expensive and more beautiful especially on a dark background as supply is less abundant than blue
Yellow although less common in black opal is not as highly sort after in a light opal and are usually paired up with orange or green
Orange is a desirable sort after colour second only to red in value
Red tones from magenta, scarlet to crimson are highly sort after especially on a black base, when this is combined with green orange and blue it becomes an extremely high and expensive quality gem
Opals can occur in many unusual patterns however there are some particular categories of patterns that can be grouped
Harlequin pattern is extremely rare and is made up of a patchwork of irregular sized squares
infire pattern is closely spaced pinpoints or specks of colour
Flash pattern shows broad irregular flashes of colour which change or disappear as the stone is moved
The most desired shape for opal is a domed cabochon oval, length is about 40% greater than the width and the height is 60 % of the width, which is quite rare to find. These types of opals are priced at a premium as so much opal is lost in the cutting. More commonly opal is cut in irregular or elongated ovals, tear drops or round to maximize the opals natural shape. Flat or excessively thick stones can devalue a stone all things being equal.
Opals under 10 carats are the most popular size. Larger sizes do not attain a higher price per carat like diamonds infact the reverse is usually the case. Opal does have a lower specific gravity than sapphires, rubies or emeralds, meaning opal will be a bigger stone size per carat than other gems
Opals with sand or potch on the back of the stone do not decrease the value unless they are visible on the face. Some darker stones can have what we call as a window in the stone where there is a transparent portion of the stone this can decrease the appeal and value