Opal is a natural hardened silica gel containing 5- 10% water. Unlike most precious stones, opals are amorphous. Some unstable amorphous structures in opal will change color or even crack due to lack of water, but some opals with very stable structures remain unchanged in normal environment. Opal belongs to opal in mineralogy, and contains two kinds of precious stones, one is colored, the other is unchangeable, and it is an amorphous silica containing water.