Opal is made from small silica spheres arranged in a regular pattern. The type of silica in Opals is chemically similar to quartz, but Opal silica contains water within the mineral structure. Precious Opal generally contains about 6-10% water. The vast number of colors in Opals are caused by the regular array of silica spheres diffracting white light, and breaking the light into the colors of the light spectrum. The actual range of color in a particular Opal is determined by the diameter and spacing of its silica spheres. The colors of an Opal can also change depending upon the angle of light incidence, so when an Opal is rotated, its color can change or disappear. This feature makes Opals extremely interesting and unique.