1. The hydrate of silica, of which there are many varieties.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• opage,
• orpah,
• opaid,
• orpan,
• cupal,
• oucl,
• ouwl,
• opan,
• opay,
• orcal,
• ormal,
• bpwl,
• optial,
• opole,
• onyl,
• ovaly,
• peopal,
• apul,
• opkay,
• bopal

• opel,
• Ophelia.

The adjective form of the word "opal" is "opalescent".

The plural of the "opal" can be the "opals".

The singular of the "opal" can be the "opal".