Opals are precious stones in which you can see vibrant colours which change as you move and look at them from different directions, in a similar way to how the colours you can see in a CD change and move as you look at them. The colour is produced by their structure not by pigments, opals are made of silica - the same thing as most sand, but it is in the form of billions of tiny spheres smaller than the wavelength of light.
Each sphere will reflect and scatter light from a slightly different place, all of these small reflections interfere with each other so in some directions for some colours they add together and in others they cancel each other out forming the beautiful colours.
Opals can be found in many types of rock and until now we didn't know what triggered them to form. Geologist Brian Senior and physicist Lewis Chatterton have studied opals very carefully using equipment such as electron microscopes and have discovered that the opal spheres seem to form in areas where silica is crystalising out of water. Normally this process cannot start in the centre of the water because there is nothing for the crystal to start to form on, so crystals grow out form the walls. But if there is Uranium decaying in the water the radiation it produces can from a nucleation point and start a crystal. All these small crystals form the centres of the spheres in the opal.
This is not just interesting science though because it means that opals are associated with radioactivity and so if you look for the radioactivity you may well find the opals.