Varieties of Opals − Doublet Opals, Triplets & More
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 and 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
Doublet Opals and Triplet Opals:
Doublet opalsare 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, which makes the translucent opal look darker once placed on the dark backing, giving it the resemblance of a black or boulder opal. The purpose of doublet opals 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.
Triplet opals Triplet opals are made by adding a clear protective quartz or crystal glass top to doublet opals; the slices are usually thinner than a doublet to make them less expensive. The domed cap protects the opal and magnifies the colour and pattern, improving the stone.
Both triplet and doublet opals should not be immersed in water or liquids, as this may cause the layers to separate.
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 light and dark shades and the colour pattern is always the same, which makes it different to real opal where the 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 when considering the quality and valuation of opals:
Intensity / Brilliance
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, and the shades of colour will be distinct, definite and intense without being cloudy. 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 a cloudy light blue to an intense electric blue.
- Green tends to be more expensive and more beautiful, especially when made into doublet opals on a dark background. The supply is less abundant than blue.
- Yellow, although less common in black opal, is not as highly sort after in a light opal. It’s usually paired up with orange or green.
- Orange is a highly sought after colour, second only to red in value.
- Red tones from magenta and scarlet to crimson are highly sought after, especially on a black base. When this is combined with green, orange and blue, it becomes an extremely expensive and high-quality gem.
Opals can occur in many unusual patterns, but 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 with a length about 40% greater than the width and the height 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 maximise the opal’s natural shape. It can also be sliced thinly, as is the case with triplet and doublet opals. 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. In fact, 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 a window in the stone, where there is a transparent portion of the stone. This can decrease the appeal and value.
To learn more about any of the above types of opals, get in touch with the expert team at Johnston Opals today.