The Collaborative International Dictionary of English v.0.48:
Opal \O"pal\, n. [L. opalus: cf. Gr. ?, Skr. upala a rock,
stone, precious stone: cf. F. opale.] (Min.)
A mineral consisting, like quartz, of silica, but inferior to
quartz in hardness and specific gravity.
[1913 Webster]
Note: The precious opal presents a peculiar play of colors
of delicate tints, and is highly esteemed as a gem. One
kind, with a varied play of color in a reddish ground,
is called the harlequin opal. The fire opal has
colors like the red and yellow of flame. Common opal
has a milky appearance. Menilite is a brown impure
variety, occurring in concretions at Menilmontant, near
Paris. Other varieties are cacholong, girasol,
hyalite, and geyserite.
[1913 Webster]