Opal Gemstones – Properties, Origins, Treatments and Buying Guide

Opal is one of the most captivating gemstones, renowned for its play-of-colour and iridescent flashes that seem to move within the stone. Unlike many gemstones whose value relies on clarity or saturation, opal’s charm lies in its unique optical effects and diverse colour patterns. Genuine opal is a prized collector’s stone with a wide range of visual appeal.

In this guide we examine opal from a professional gemmological and commercial perspective, covering its physical properties, crystal structure, treatments, origins and how to identify high-quality stones.

Opal is a hydrated amorphous form of silica that produces a remarkable display of colour known as “play-of-colour.” Unlike crystalline gemstones, opal does not have a regular crystal lattice, which gives it its distinctive optical effects.

Opal forms in cavities and fractures in sedimentary rocks, often replacing limestone or volcanic material. High-quality opal can display vivid flashes of red, green, blue and other colours across the surface.

Opal’s chemical formula is:

SiO₂·nH₂O

This means it is composed primarily of:

Silicon dioxide (SiO₂)

Water (3–21% by weight, typically 6–10%)

The water content and microscopic silica spheres produce the characteristic play-of-colour.

Colour intensity and pattern are influenced by:

Size and arrangement of silica spheres

Thickness of layers within the stone

High-quality opal displays vibrant colour flashes across the entire gem.

Opal belongs to the silica mineral group, though it is amorphous rather than crystalline.

It does not have a defined crystal system, which contributes to its unique optical properties and internal diffraction of light.

Opal is valued for its interplay of colour, transparency and unique brilliance, making it a highly sought-after gemstone for jewellery and collectors.

Lustre
Opal has a vitreous to sub-vitreous lustre, giving it a soft glow and shimmer.

Transparency
Ranging from transparent to opaque, depending on variety and thickness. Precious opal is usually semi-transparent to translucent to allow play-of-colour to show.

Refractive index
1.37 – 1.52, providing a soft brilliance and enhancing internal colour diffraction.

Specific gravity
1.98 – 2.25, relatively light compared with many gemstones.

Opal occurs in a wide variety of colours:

White / light opal

Black opal

Crystal opal

Fire opal (orange to red body colour)

Blue and green flashes

The most prized opals display strong play-of-colour across the surface, with red flashes being particularly valuable.

Opal has a Mohs hardness of 5.5–6.5, making it relatively soft.

It is fragile and can chip or crack easily, so protective settings and careful handling are recommended for rings and daily wear jewellery.

Many opals are sold untreated, showing natural play-of-colour.

Occasionally treatments include:

Sugar-acid or smoke treatment to darken body colour

Stabilisation with resin or backing for doublets and triplets

High-quality solid opal is usually completely natural and untreated.

Avoid ultrasonic and steam cleaners, harsh chemicals, and sudden temperature changes.
Clean opal gently with warm water, mild soap and a soft cloth.
Store separately from harder gemstones to prevent scratches.

Major sources of opal include:

Australia (Lightning Ridge, Coober Pedy)

Ethiopia

Mexico

Brazil

Honduras

Australia produces the finest black and crystal opals, highly valued in the jewellery market.