Opal Grading

Colour Play and Pattern – Australian Opal

Body tone, colour play and pattern are all equally important in determining an opal’s value.  A black opal showing a very bright, red play of colour in a special pattern is the most valuable.  The thickness of the opal layer is also a consideration, as is the beauty of the patterning, the cut, weight and finish.

  • Composition: Hydrated Silica, SiO2 nH2O (Silicon Dioxide)
  • Hardness: 5.5 to 6.5 or harder
  • Fracture: Conchoidal
  • Cleavage: good to fair (for example, bolder seam opal or Yowah nut opal splits almost in a mirror image)
  • Lustre: subvitreous to resinous
  • Specific gravity: 1.9 to 2.3
  • Streak: white
  • Diaphaneity: transparent to opaque

Different opals have differences in mineral composition depending on the geological environment from which they originate.  

Opal varieties are determined by the specimen’s origin and colour.

Types of Opal

  • Gem or Precious opal: is defined by the brilliance of colours in the colour bar. If it is on flawless potch, it is considered more valuable than on a mixed layer of potch.
  • Black opal: shows a bar of strong brilliant colours of the spectrum on a flawless black potch background. The definition of black opal is a bar of gem colours over a black or dark background of potch. In fact, any colour bar of opal on black or blue/black potch is black opal.
  • Black crystal opal: contains gem colours over a smoky, transparent background. Some people regard it as the best type of opal.
  • Semi-black opal: has gem colours on greyish potch backing, not in the same league as true black opal.
  • Grey opal: varies in density from light grey to nearly black and is sometimes smoky in appearance. The colours can be very bright on dense, semi-dark backings of potch and can often be passed off to the unwary as black opal.
  • Crystal opal: strong, dazzling colors in a more transparent body of light or crystal opal.
  • Milky or white opal: gem opal with all the usal colours on a milky white background.
  • Light opal: is a gem opal on a light to translucent background containing colour.
  • Boulder opal: there are two varieties – matrix boulder and vein boulder, with many sub-varieties between the two, such as nut opal (known as Yowah nut), spud and pancake boulder opals.
    • Matrix boulder opal: a porous sedimentary ironstone having minute cavities filled with precious opal. when cabochon cut, the dark brown ironstone sparkles with vivid pin flecks of colour. It is a very attractive stone.
    • Vein boulder opal: ironstone, mud-stone and sandstone boulders containing veins of opal from millimetres to centimetres in thickness.
  • Opalised wood: wood replaced by opal, can be precious or non precious.
  • Fossil opal: shells, animal bones, plants and a host of other pseudomorph types replaced in part or whole with opal. Again, it can be precious or non-precious.
  • Common Opal
    • Potch: common opal with not play of colour. Fond in many localities throughout the world. About 95% of potch is associated with precious opal depending on each locality. It varies from black to milk white to greenish, yellowish or bluish in colour
  • Note: the term common opal means other types of opal of lesser commercial value and not necessarily of common occurrence. In fact some are very rare, sch as hydrophane.
  • Opalite: green or yellow common opal.
  • Moss opal: jelly like common opal with intrusions that resemble moss in appearance and coloour.
  • Hydrophane: opal with some gem colour that shows when soaked in water. Ohers lose colour when put in water.
  • Jelly opal: a variety of opal highly translucent with a weak colour play.
  • Crystal system: opal contains small crystals of a naturally ocuring form of silica known a crystobalite.
  • Fire opal: hyacinth red to honey yellow colours with firelike reflections.
  • Girasol: bluish white, translucent, with reddish reflections in bright light.
  • Sun flash: translucent, transmits deep colour in sunlight.
  • Picture stones: where an image appears in the bedrock of a potential opal

Patterns in Opal

Patterns are a very important aspect in the value of opals and these come in a brilliant variety of natures own design, exclusive to opals.

  • Harlequin pattern: is surely at the top of the list and the most prized of all brilliant opal patterns. Such rarities portray spangles of rounded, angular to roughly square patches, presenting a harlequin appearance of interchanging colours. They resemble the colourful spangled costume worn by clowns
  • Hexagon pattern: rarely seen in opal, is in an interlocking design of continuous stacking in vivid colours.
  • Ribbon: resembling ribbons of changing colours across the stone, which can be in wavy bands or straight thin lines.
  • Pinfire: seen as pinhead spots, the most outstanding being a colour unit, perhaps red, radiating one way, which changes to another colour, perhaps green, when turned at various angles. Some pinfire colours blend into peacock tail design, either on part of the stone or from a central nucleus. Other pinfire colours blend into fern or foliage design.
  • Stardust: has lovely speckled points of multi colour.
  • Flag pattern: colours arranged in three cornered flagstone design. Some patterns resemble fish scales.
  • Chinese writing: this pattern is rare and valuable. The colours form a design appearing like Chinese writing.
  • Chaff pattern: exhibits colour segments like chopped chaff or multiple situated parallel lines.
  • Straw pattern: irregular, flat, straw like design units overlapping one another.
  • Grass pattern: thinner nits of color in blades of grass effect. The green colour makes a true grass pattern.
  • Mackeral sunset: presents broken streaks of colour spectrum, likened to sunset through clouds.
  • Butterfly opal: has colour arrangement formed like patterns of butterfly wings.
  • Mossy pattern: a splotchy coloured sheen in moss effect.
  • Floral pattern: a flashy effect like a bunch of flowers, seen frequently in Queensland opal.
  • Rolling flash: brilliant colors rolling across the face of opal, some having a cats eye, rolling effect. Green is especially striking rolling across the stone.
  • Fiery opal (or Red Fire): predominantly red play of colour sometimes seen as pinfire or stardust flashes.
  • Star pattern: has a six pointed star directional effect floating across the face of the opal and is very rare.
  • Clover leaf: basically green in the form of clover leaf interspersed with blue.
  • Broken flash: different parts of the stone display a broad colour flash upon rotation.
  • Flame opal: as in the ‘Flame Queen’ from Lightning Ridge, has a bushfire flame-like pattern across the face of the opal.
  • Rainbow opal: the colours are positioned like a rainbow.

In cretaceous sediments, opal occurs in thin or thick horizontal and vertical veins as happens in White Cliffs, Andamooka, Coober Pedy and Mintabie. In Queensland, Boulder opal is frequently found in vertical seams. The horizontal veins are more important than the verticals. Irregular nodules or Nobbies are characteristic of Lightning Ridge along with seam opal.

Reference: Aracic S, 1999, Redisocer Opals – in Australia, Kingswood Press Pty Ltd, Qld Australia – page: 89 – 94

Grading: Colour and Pattern (example)

DETAILS

ID Number: 201535

Category: Unsiet Opal

Opal Type: Solid Boulder Opal

Colours: Blue, Green, Orange, Red, Yellow

Weight: 3.48ct

Width: 8.9mm

Length: 10.3mm

Origin: Koroit, QLD, Australia

Brightness: Brilliant

Cabochon: Low Cabochon

Pattern: Flagstone

Shape: Oval

Transparency: Opaque

Thickness: 4.1

NOTES:

Hardness of opal: The hardness of opal is measured on Moh’s scale and is judged on abrasiveness.

Example: on a scale of 1 to 10 – talc sits at 1, and glass sits at 5 while diamonds sit at 10 being the hardest substance. Opal sits between 5.5 and 6.5 being closer to the hardness of quartz. Although glass sits at 5 it is brittle and easily shattered. Opal is harder than glass, because opal is formed from silica, its composition is different to glass. The structure of opal is of conchoidal fracture. The difference being that glass is soft and brittle while opal is soft but not so brittle.

Australian gem opal is fairly hard and just as tough as it is beautiful as it withstands a lot of rough handling during the process from being mined out of the ground to finally being presented as an art piece. Australian opal has a higher water content making it the strongest opal in the world. Opal has no definite cleavage in comparison to a diamond while although they are the hardest substance known a diamond is also quite brittle – if hit with a chisel across instead of along its cleavage it will shatter. (note; there is an exception to this rule of thumb with boulder opal which can still be split apart because the opal while still liquid, formed in between cracks and crevices of the boulder. They can be termed mirror images and are termed splits).

Cleavage is the tendency of some minerals to break along flat surfaces and fracture is when some minerals to break along a curved or irregular surface. Opal has no cleavage it has the fracture type of chonchoidal. So Opal usually is broken on a curved or irregular surfaces.

Conchoidal fracture breakage that resembles the concentric ripples of a mussel shell. It often occurs in amorphous or fine-grained minerals such as flint, opal or obsidian, but may also occur in crystalline minerals such as quartz.

In summary

The value of an opal is determined by;

  • The type of opal
  • The predominant colours it exhibits
  • The clarity or brilliance of these colours
  • The patterns in which the colours are arrayed
  • Good patterns of the diffracted colours have an enormous impact on the value of the opal
  • Pinfire and small type patterns are more common, and thus less expensive than broad patterns or large flashes
  • Distinct patterns such as rolling flash, straw pattern, Chinese writing, ribbon, and especially harlequin, are very rare and considered collectors’ items.