An Etymological Dictionary of Astronomy and Astrophysics

A → mineral, an amorphous form of → silica, SiO₂ with some → water of → hydration,
found in many varieties and colors, including a form that is milky white.

Etymology (EN): M.E., from M.Fr. opalle, from L. opalus, from Gk. opallios “opal, gem;” probably from a source akin to Skt. upala “jewel, precious stone.”

Etymology (PE): Loan from E., as above.