If you have ever hold an opal in your hands then you will know the feeling you get in looking intently at the breathtaking play of colours, patterns and reflections thet the gem displays, a scientifically esaplained harmony.
Precious opal in fact is composed of tiny spheres of hydrated silica arranged three-dimensionally. When white light meets the spherules of silica the light is diffracted in different directions and assumes different colours of the rainbow, depending on the size of the spherules. If the silica spherules are small (about 0.20 micron), the observer perceives a blue colour, but if the size increases then the colours tturn into green (about 0,25 micron) and then into red (about 0.32 micron).
Both the colours intensity and brightness of a gemstone depend upon a number of factors: the arrangements of sets of spherules of the same colour, the degree of uniformity and the regularity of the surface of the gem. The background colour instead is due to traces of Ni, Cr, Fe, Mn and inclusions of other minerals.