When opal is formed, deep under the earth, the silica which is deposited as the water evaporates is in actuality microscopic spheres. Any spheres, when packed together are going to leave spaces between them. It is the gaps that create the colour. Hold an opal in the sunlight for the best play of colour. The light bends between the spheres and is diffracted, which splits white light into the colours of the rainbow.

We all recall our childhood schooling where they seemed to think that teaching us how to remember the colours of the rainbow was important. ROYWBIV. People remember this in various ways, in the UK, it went like this. Richard of York Gained Battles In Vain. What this means is that the larger the spheres, the closer to the Richard end of the spectrum and we see red colour in the stone, and the smaller the sphere indicates that we are lower down the scale and we see our blues and greens.