noun

• (mineralogy) A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity, of the chemical formula SiO₂·nH₂O.
  "Although there are dozens of different types of gems, among the best known and most important are […] . (Common gem materials not addressed in this article include amber, amethyst, chalcedony, garnet, lazurite, malachite, opals, peridot, rhodonite, spinel, tourmaline, turquoise and zircon.)"
• (mineralogy) A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity, of the chemical formula SiO₂·nH₂O.
• (biology, genetics, biochemistry) A colloquial name used in molecular biology referring to a particular stop codon sequence, "UGA."

name

• A unisex given name from English from the precious stone, invented in the nineteenth century.
• (Australia) A type of petrol made by British Petroleum designed to be unable to be used for petrol sniffing.
  "Halton says, for example, that if there are bylaws that prohibit trafficking in fuel in a community where the non-sniffable petrol, Opal, had been introduced it would be the job of the local police to enforce the law. Her department, on the other hand, was dealing with the issue of the availability of Opal."