Opals are precious stones in which one can see vibrant colours which change as you move and look at them from different directions. Opal spheres seem to form in areas where silica is crystallising 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 from 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.