How Opal Color is produced
Ok, this article on opal color is not an earth science, geology, or chemistry lesson, but since many folks wonder what makes opals glow in a rainbow of colors, here’s a quick explanation that’s hopefully not too technical. Just in case some of the terms are unfamiliar, there’s Glossary of Terms following the text.
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 colors of the spectrum are diffracted. So it is a combination of deflection (bending) and diffraction (breaking up) of light rays that creates the color in opal. If you move the stone, light hits the spheres from different angles and bring about a change in color. The name opal actually means “to see a change in color.” The way in which opal color changes within a particular stone as it is rotated and tilted is called the stone’s play of color as you can see featured in these amazing opal pendants.
How opal color is defined
The size of the spheres has a bearing on the opal color 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:
Shapeless. Not consisting of crystals. Non crystalline. Glass is amorphous. Sugar is crystalline.
The bending of rays of light from a straight line.
The Breaking up of a ray of light into either a series of light and dark bands, or into colored bands of the spectrum.
To spread out so as to cover a larger space or surface. To scatter.
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 colors in opal.
A compound produced when certain substances chemically combine with water.
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 opal color.
Opal comes from the Latin word opalus which means to see a change in color. (that is, opal color) Chemically, opal is hydrated silica, similar to quartz. (use graphic 1168 to show change of color in the three images)
Opalescence means a play of opal color
A play of color, similar to that of an opal.
Not allowing light to pass through. The opposite of transparent.
Play of Colour
The way in which colors change as an opal is tilted in different directions.
(Silicon Dioxide) A hard, white or colorless 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.
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 colors are: red, orange, yellow, green blue, indigo, and violet.
A round three dimensional geometric shape whose surface is equally distant at all points from the centre point.
Letting light through without being transparent. An object that lets some light pass through
Easily seen through.(glass like) it means that you can see through it. if you take a clear glass plate and put it to your face, you can still objects on the other side.
Please leave a comment if you have any further questions about opal color.