What is the opals crystals system?

Opal, being amorphous, is not really 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. Many scientific groups and references, including the acclaimed Dana's System of Mineralogy, categorize Opal together with the "true" minerals. For this reason, Opal -- as well as other amorphous types that fall under the definition of a mineraloid.

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