How does opal classification work?

If you have seen our online opal catalogue, you will have noticed a number of categories used to describe and classify each stone. This system of offering detailed information about each stone is based on the Australian Opal Ebusiness Association’s (AOEA) Opal Classification Standard, and aims to offer as much information about each stone as possible, allowing you to make an informed decision about your opal.

Clearly there is no substitute for viewing a stone in person, however we believe that our videos and pictures (in which we take meticulous care to represent our stones accurately), combined with this classification system, allows our customers to buy with confidence. Our clients are almost always pleased with their stone once it arrives, since as a general rule a stone is better in real life than in any photograph. For a good detailed run-down on how the value of an opal is determined, please see our article on valuing opals.

Our Classifications Explained:

ID – This is the unique identification number issued to each opal when it is processed.

Category – The preface Solid means that the stone is a natural cut & polished opal which does not have any kind of backing adhered to the stone to enhance the colour (as is the case with the partially fabricated stones – doublets or triplets). Queensland Boulder Opals, even though they have a natural brown ironstone backing which makes the stone darker, are still known as solid opals since this is the natural formation of the stone. Read more about the types of stones.

Black Opal refers to opal which has a dark grey to black body tone, and is generally mined in the Lightning Ridge area of New South Wales. As a general rule, black opal is the most valuable form of opal, since its dark body tone causes the colours to be more vibrant.

Boulder Opal is opal mined in Western Queensland which normally has a natural brown ironstone backing attached to the stone. Boulder opal usually has a very dark body tone and is thus generally the second most valuable form of opal.

Crystal Opal means any kind of opal which has a translucent or transparent quality (i.e. you can see through it). Translucent or transparent stones often have an enhanced clarity of colour, and for this reason it usually increases the value of a stone. The term Crystal Opal normally denotes opal with a very light body tone, however Black Crystal Opal refers to a crystal opal which has a dark body tone.

Semi-Black Opal refers to opal which has a light to medium grey body tone and is therefore not quite dark enough to be called black opal. These opals usually fall within the ‘dark opal’ category in the Body Tone Index. (See diagram further down) Semi black opal is generally found in Lightning Ridge, but is also found in White Cliffs and occasionally South Australia. This can be one of the lesser valuable forms of opal.

White Opal means opal with a white to light body tone, and is also known as milky opal. White opal is found in large quantities in South Australia, and the bulk of it does not have the same vibrancy of colour as found in other forms of opal. For this reason, it is generally one of the least valuable forms of opals. (High quality white opal is available however.)

Setting – In the case of jewelry, this indicates the carat of the gold used, and whether it is White Gold or Yellow Gold.

Weight – This refers to the carat weight of the stone. Five carats equals approximately one gram. The value of an opal is usually determined by calculate a price ‘per carat’ according to the colour and appearance of the stone, and this value is multiplied by the carat weight. When there are multiple stones, the carat weight of all stones combined is given.

Origin – The place in Australia where the stone was mined. See our article on Australia’s opal mining fields for more detailed information on each field.

Dimensions – The dimensions (width and length of the stone facing upwards) measured in millimetres. One inch equals 25.4 millimetres. In the case of a freeshape stone, the measurements are generally given at the widest points of the stone. When there are multiple stones, the dimensions of the largest stone are given.

Thickness – The measurement in millimetres of the stone’s thickness (i.e. looking at the side of the stone). This measurement is taken as close to the centre of the stone as possible. This includes any potch (colourless opal) or ironstone which is naturally attached to the back of the stone. In the case of boulder opal, the actual layer of opal can be less than 1mm thick. Our stones are cut with enough backing on them to support and stabilise the stone and give them a good shape. We never leave extra weight or thickness on the back of a stone to boost its carat weight or value.

Body Tone Index – (See Figure 1, below). This is a device used to classify the darkness of a stone. Generally a darker stone leads to more vibrancy of colour, however it depends on the individual opal. Boulder opals are always listed as having a body tone index of 0 since they cannot be classed in the same system as other opals due to their ironstone backing. Boulder opals generally have a very dark body tone however.

Transparency – Refers to the ‘diaphaneity’ (transparency) of an opal. Opaque means the stone is not transparent. Translucent means the opal has a semi-transparent nature. Transparent means you can see through the stone. This category is used to determine whether a stone has any of the properties of a crystal opal.

Shape – Refers to the shape of the stone. Freeshape means anything which is not in a standard oval shape. Cabochon refers to the dome on the top of the stone. A cabochon can effect the appearance and pattern of a stone – for example, crystal opals often look better with high cabochon, whereas black opals can look better with a low to medium cabochon. This is up to the individual stone however and relies on the skill of the opal cutter to maximise its beauty and pattern. Low Cabochon means it has a flat or hardly any dome. Medium Cabochon means it has a medium dome. High Cabochon means it has a high dome on the surface.

Colours – Lists each colour of the spectrum which is visible in the opal. Generally the most prominent colour is listed first, then the second most prominent colour and so forth. The rarity of colours is as follows – in order of the rarest (most valuable) to the most common (least valuable). Red; Orange; Yellow; Green; Blue. Red is therefore the rarest and most highly sought-after colour in an opal, and therefore fetches the highest price. Unusual colours may also occur, such as purple and aqua which can also enhance the beauty and value of a stone. Read more about how opal displays colours in our opal colour article.

Brightness – This is one of the most important ways in which we determine the value of an opal. There are three brightness ratings – Subdued, Bright, and Brilliant. These categories are quite broad and are intended to give a general indication of a stone’s brilliance. Subdued means the stone falls into this category with the least brightness – (note that it still may be a beautiful stone). Bright means the stone has a good level of brilliance and falls into the middle category. Brilliant means the stone is a real eye-catcher – has excellent brightness and falls into the top category in this classification. Brilliant is obviously the most sought-after and valuable property for a stone as it is a very desirable quality.

Pattern – This is a description of the arrangement of the colours on the face of the stone and how they appear to the eye. The most common pattern is Floral, which we use as a very broad description meaning a random and relatively indistinct pattern of opal colours. Most opals fall into this category.

More valuable patterns include Pinfire (small dots of colour sparkling like stars), Broad Flash (large sections of colour which flash brightly at certain angles), and Rolling Flash (a large section of colour in which a bright flash rolls across a section of the stone as you move it).

Even more valuable patterns include; Ribbon (Almost indescribable – Multiple rolling flashes which line up in different sections moving next to each other and in succession), Flagstone (large distinct blocks of colour), Straw (small and thin multiple lines of colour next to each other), Chinese Writing (thin strokes of colour which look like chinese writing).

The most valuable and rare pattern is Harlequin, in which blocks of colour lie next to each other and are of approximately the same size and shape (like a checkerboard). This pattern is extremely rare and is the legendary in opal circles. Many websites on the internet use the term Harlequin very liberally, so be wary of what you are buying. Traditionally a true Harlequin opal is extremely valuable and rare, and can fetch hundreds of thousands of dollars. In two decades of experience, our opal cutter has only ever seen two true Harlequin opals, just to give you an idea of their rarity.

Notes – This is where any extra description, special characteristics, faults or interesting attributes are mentioned.

Any questions? Don’t hesitate to email us!