Fireworks. Jellyfish. Galaxies. Lightning. Opal’s shifting play of kaleidoscopic colours is unlike any other gem.

Opal forms in regions like Australia’s semi-desert “outback” as a result of seasonal rains soaking into dry ground. The rainwater, carrying dissolved silica (a compound of silicon and oxygen), seeps deep into ancient rock formations. When the weather dries out, much of the water evaporates, leaving behind solid deposits of silica that eventually become opal.

How Opal Forms

Opal is renowned for its striking play-of-colour, a phenomenon where flashing rainbow colours appear as light diffracts through the stone. Opal is classified into two main types: precious and common. Precious opal exhibits play-of-colour, whereas common opal does not.

This play-of-colour occurs in precious opal due to its structure of sub-microscopic spheres arranged in a grid-like pattern—similar to layers of Ping-Pong balls. As light waves travel between these spheres, they bend and split into the colours of the rainbow, creating the play-of-colour effect. The colours seen depend on the size of the spheres: spheres around 0.1 micron in diameter produce violet, while those about 0.2 microns produce red, with intermediate sizes creating the other colours of the spectrum.

Types of Opal

Opals are categorized into several types, with the main five being:

• White or Light Opal: Translucent to semi-translucent with play-of-colour against a white or light grey background, known as bodycolor.

• Black Opal: Translucent to opaque with play-of-colour set against a black or dark background.

• Fire Opal: Transparent to translucent with body colours in brown, yellow, orange, or red. Often lacking play-of-colour, it is also referred to as “Mexican opal.”

• Boulder Opal: Translucent to opaque with play-of-colour on a light to dark background. Fragments of the surrounding rock, or matrix, are incorporated into the gemstone.

• Crystal or Water Opal: Transparent to semi-transparent with a clear background, this type showcases exceptional play-of-colour.