Opal as a Gemstone

What is opal and how did it form?

In simple terms, opal is a mix of silica and water, more precisely a hydrated amorphous form of silica (SiO2·nH2O). It is similar to glass in hardness and other properties. Water content of Australian opal is very low and the water molecules are locked into tiny voids within the opal’s crystalline silicon structure, therefore, Australian opals are very stable. Most of the opals found from other parts of the world contain much higher percentages of water, up to 30 percent, that makes them prone to cracking when dry. Furthermore, some opals from outside Australia are very pourous and lose their colours even for months when subjected to water. Australian Opal, on the other hand, is non-porous and therefore impervious – it cannot soak up water, oil, or anything else. The stability of Australian opal makes it significantly more valuable and also more usable than other opals.

Australian precious opals formed in very special conditions about 100 million years ago, when the great Eromanga Sea, that covered central Australia, started to dry out. Highly acidic water dissolved silica from sandstones. This silica-rich solution ran into cracks and voids, caused by natural faults or decomposing fossils on the drying seabed. As the water evaporated, it left behind silica deposits that eventually opalized. These circumstances were different from some of the world’s other opal fields, where opal has formed in a volcanic setting, not the sedimentary one seen in Australia.

Where opal formed in cavities, left by parts of living things, buried in the sand or clay, for example wood, bones, shells or pinecones, the opal formed fossil replicas of them. A fossil is simply “the remains or traces of an ancient animal or plant preserved in rock”. Opalised fossils formed in similar ways to other fossils, except that they are preserved in silica. Elsewhere, fossils are preserved in minerals such as agate, pyrite or limestone. We often come across these fossils in rough opal, especially wood and shells.

Opalisoitunut belemniitti
Opalized belemnite, Coober Pedy Australia
Opalisoitunut kotilo, Coober Pedy Australia
Opalized gastropod, Coober Pedy Australia

Brightness, colours and body tone

Vividness (also called brilliance or brightness) of colours has a majour influence on the value of an opal. The best gem-grade opals have so bright colour that they seem to pop out from the stone and shine bright in any type of lighting conditions. Subdued opals, on the other hand, usually only display their colours when viewed close up under direct light.

There are two different scales used to classify the brightness of an opal. On one scale the brightness has been divided to seven categories (B1-B7), B1 being the brightest. On the other scale the brightness is determined on a scale from B1 to B5, B5 being the brightest. The brightess classification is always a subjective opinion and therefore, we prefere to describe the brightess in words. The brightness of an opal on the five categories scale can be described as:

  • B1, subdued / faint: Opal shows play-of-colour only under sunlight.
  • B2, moderately bright / dull: Opal shows some dull play-of-colour under sunlight or spotlight.
  • B3, bright: Opal shows a nice play-of-colour under sunlight or spotlight.
  • B4, very bright: Opal shows a nice play-of-colour under low light and a very good under sunlight or spotlight.
  • B5, vivid / brilliant:  Opal shows a bright crisp mirror-like play-of-colour under sunlight or spotlight and often seems even brighter in subdued light.
Very bright crystal opal that reflects the colour to the holder's fingers.

The most common colours on opals are blue and green, whereas red is the rarest. The colours can form patterns on the stones. Many of these patterns have been given a descriptive name, such as Pinfire where the colour has formed as tiny coloured dots, Broad Flash with big patches of colour and the rare Harlequin patterns where the colours form a mosaic-like pattern on top of the stone. A true Harlequin pattern is a repeating pattern of contracting diamonds or elongated squares.

The colour on an opal usually changes when the stone is moved around. For example, parts of the colour or areas of colour may only show from certain directions or the colour may move around, rolling across the stone. This play of colour is what makes opal truly a unique gemstone!

The cause of colours and play of colour in opal are diffractions of light caused by regular stacking of nanometer-sized silica spheres, similar to the stacking of atoms in crystal structures. Therefore, the direction, type and amount of light effect the colours we see in an opal. When setting opals to jewellery, it’s important to consider this as the setting position and direction may have a significant effect on the aesthetics of the final product!

Extremely rare harlequin pattern black opal

A dark opal with vivid clover leaf pattern.

Black opal with Pinfire pattern.

A body tone scale has been developed to grade the black opals. Only opals that have the body tone of N1 to N4 can be called black opals. The body tone is determined by holding the stone face up on the scale and compairing its overall body tone to the shades on the scale.

The body tone, however, is only one factor that influences the value of a black opal. It must be noted as well that the body tone scale as a valuation tool only applies to black and dark opals. White opals and their body tone should be graded differently. Being on the other end of the scale doesn’t mean white opal is lower quality than black opal.

Black Opal Body Tone Scale

The most sought after body tone on black opals is naturally jet black N1 that brings out the colours of the stone and makes them glow. It must be noted that the body tone is only one factor that should be considered when valuing an opal. Especially the brightness of the stone, colour play and pattern are factors that are even more important factors than the body tone when the value of an opal is determined. For example, from the two opals below the lighter based N4 body tone black opal is more valuable as it has more vivid multi-colour colour play and plenty of red.

Body tone N1 black opal
Body tone N4 black opal

The body tone of white and very light opals is assessed based on the purity and over all aesthetics of the stones body tone.

The body tone should be attractive and it should bring out the colours of the stone nicely. If the body tone appears dirty or is uneven, it often lowers the value of the stone. On the other hand, beautiful tones of white, such as the pearl white body tone of the stone on the video, increase the value.

White opal from Coober Pedy that has a lovely pearl white body tone

Inclusions and defects

Opal is a natural stone and therefore opals often have inclusions or defects. These often lower the value of the stone, but they also may add to the value, if an inclusion is aesthetically pleasing and doesn’t affect the usability of the stone in jewellery. Inclusions can be very attractive and make a stone totally unique.


Sandstone on top of the opal is always a defect. Sandstone can’t be polished and often can’t withstand wear like opal does. Small sand spots on the face of the opal that are invisible to the naked eye,  or very hard to notice, usually don’t affect the usability of the stone. On the bottom of the stone sand can even be desirable, when not visible to the face of the stone, as it shows that the stone is a natural opal. The opal below has visible sand on the face of the stone.

Colourless areas and potch on the face

The opal above has colourless areas, especially on one side of the stone. Colourless areas and potch, colourless common opal, on the face of the stone usually lower the value of it. However, potch on the stone can make the stone very interesting and can even increase the value. At the end of the day, it’s all about aesthetics of the stone.

Fractures and cracks

All fractures and cracks in the opal are considered as faults. Fractures make the stone unstable and the stone may completely break for example from changes in temperature. Australian opal in general is very stable and is not prone to cracking after it’s been cut, unlike opals from other parts of the world.