Opal is particularly famous because of its very diverse shapes and forms. It has always been the subject and inspiration of many artists and writers. Originally, the name of the stone came from the Greek word Opallio, which means "perceive a change in color", referring to its ability to change color when reflecting light. Because of their special functions, opals are also known as expensive, and the rarest ones are dark tones.
The formation of opal is a completely natural but long process.
The substance itself is actually a solution of water and silica. Silica is transported and absorbed from sandstone by water, and then it is carried into various voids and cracks. These cracks are usually fossils that are decomposing, and sometimes they are just natural faults.
The water that carries these silica elements will eventually evaporate, leaving behind all the silica.