The solution of water and silicon dioxide formed Opal. When the water runs down through the earth, it collects silica from sandstone and carries this silica-rich solution into gaps and cracks, which is caused by decomposing fossils and natural disasters, and then the water evaporates, it left silica deposits behind. This cycle repeats over a long period, and finally, Opal is formed. Inside of an opal, visualize like a bag of marbles. The marbles are the silica spheres.