[Early Modern English, from Middle French opale, from Latin opalus, alteration of Greek opallios, probably from Sanskrit upalaḥ, stone, precious stone (often in the sandhi form opalaḥ in the names of gems such as nīlopalaḥ, sapphire : nīla-, blue + upalaḥ), possibly variant of upara-, lower, from upa, towards, under, down; see upo in the Appendix of Indo-European roots.]