Opal

Opal, being amorphous, is not truly a mineral but a mineraloid. One of the scientifically accepted standards defining a mineral is that a mineral must have a crystal structure, which opal lacks. It is most abundant in volcanic rocks, especially in areas of hot-spring activity. It also forms pseudomorphs after wood and other fossil organic matter and after gypsum, calcite, feldspars, and many other minerals that it has replaced.