Why And How Is Opal Coloured?

As the silica in solution was deposited, and the water content gradually decreased, spheres formed in the gel. The spheres are formed by the particles of silica spontaneously adhering to other particles which form around it. These spheres of amorphous silica range in size from 1500 to 3500 angstroms (1 angstrom is 1 ten millionth of 1 millimetre).

The spheres are not only remarkably uniform in size but are packed, in gem quality opal, in a very regular array. Because they are spherical, there are tiny holes remaining in the structure (much the same as when marbles are placed together in a container) and these holes too are arranged in a regular three dimensional way. Therefore because of the regular array of these cavities, opal is an optical diffraction grating for visible light.   

When the spheres are bigger (about 3500 angstroms diameter) the red or orange colours are produced. And at the other end of the scale, at about 1500 angstroms diameter, the blue end of the spectrum is diffracted. Between these figures the rest of the colours of the rainbow occur.  From this it can be deduced that the light diffraction in the voids is greatest when the sphere size is greatest. Therefore red is usually the brightest colour and the blue duller. 

In summary, the colour in precious opal is caused by the regular array of silica spheres and voids diffracting white light, and breaking it into the colours of the spectrum. The diameter and spacing of the spheres controls the colour range of an opal. Small spheres produce opal of blue colour only (the most common), whereas larger spheres produce red (the rarest colour).  For more detail, see our article on how colour is formed in opals.