Opalize refers to the process of transforming into opal. However, there are several other words that can be used to describe this process. Synonyms for opalize include: mineralize, petrify, fossilize, calcify, lapidify, harden and solidify. These words all describe a transformation from one state to another, specifically the changing of a material into a more solid and durable form. While opalize specifically refers to the transformation into opal, the other synonyms can be used more broadly to describe the process of any material becoming harder and more permanent.