How Opal Colour is Produced

It took the development of the electron microscope to work this out. Precious opal is made up of tiny uniform spheres of transparent hard silica, which fit together in an orderly three dimensional frame, sitting in a "bath" of silica solution. It is the orderliness of the spheres that separates precious opal from common opal.

Light passes through the transparent spheres in a direct line, but when it hits the 'bath' of silica, it is bent and deflected at different angles, thus producing a rainbow effect.

Deflection & Diffraction

Depending on the size of the spheres, varying colours of the spectrum are diffracted. So it is a combination of deflection (bending) and diffraction (breaking up) of light rays that creates the colour in opal. If you move the stone, light hits the spheres from different angles and bring about a change in colour. The name opal actually means "to see a change in colour." The way in which colours change within a particular stone as it is rotated and tilted is called the stone's play of colour.

How colour is defined.

The size of the spheres has a bearing on the colour produced. Smaller spheres bring out the blues, from one end of the spectrum. Larger spheres produce the reds from the other end. The more uniform the spheres are placed, the more intense, brilliant and defined the colour will be.

Glossary of Terms:

Amorphous

Shapeless. Not consisting of crystals. Non crystalline. Glass is amorphous. Sugar is crystalline.

Deflection

The bending of rays of light from a straight line.

Diffraction

The Breaking up of a ray of light into either a series of light and dark bands, or into coloured bands of the spectrum.

Diffuse

To spread out so as to cover a larger space or surface. To scatter.

Fluorescent

A light produced by the electrical stimulation of a gas or vapour. Fluorescent lights have a similar effect on opal as a bright cloudy day--they do not properly bring out the colours in opal

Hydrate

A compound produced when certain substances chemically combine with water.

Incandescent

Glowing with heat (red or white hot) as in a light bulb which glows white hot, but produces a light that more closely simulates natural sunlight. Sunlight and incandescent lights bring out the natural colours in opal.

Opal

Opal comes from the Latin word opalus which means to see a change in colour. Chemically, opal is hydrated silica, similar to quartz.

Opalescence

A play of colour, similar to that of an opal.

Opaque

Not allowing light to pass through. The opposite of transparent.

Play of Colour

The way in which colours change as an opal is tilted in different directions.

Silica

(Silicon Dioxide) A hard, white or colourless substance, that in the form of quartz, enters into the composition of many rocks and is contained in sponges and certain plants. The needle in the mouth of a female mosquito is made of silica. Flint, sand, chalcedony, and opal are examples of silica in different forms.

Spectrum

The band of colours formed when a beam of white light passes through a prism or by some other means (e.g. mist or spray, in the case of a rainbow) The full range of spectrum colours are: red, orange, yellow, green blue, indigo, and violet.

Sphere

A round three dimensional geometric shape whose surface is equally distant at all points from the centre point.

Translucent

Letting light through without being transparent.

Transparent

Easily seen through.(glass like)