An opal's story begins with silica dissolving into groundwater from the surrounding strata. Over millions of years, some of the water was lost by evaporation through the permeable rock around it. The silica also had the natural tendency to try and reduce its contact with water. Thus, it packed together as tightly as possible, eventually developing into aggregates, forming, with the water, what is known as a 'sol'. The viscosity of this sol determined what size the aggregates needed to be before settling out of solution. Hence aggregates of similar size tended to settle together. Once these settled, they formed a gel, which gradually lost more water, and cemented together to form a hard mineral. In most cases, there was too much cementation, creating a continuous substance. However, sometimes there was just the right amount, so that the stone hardened, but there were gaps left in between the silica spheres. These gaps acted as a diffraction grating, splitting incident light into its full spectrum. This is what causes the brilliant multi-hued colour play seen in precious opals.