Definify.com
Webster 1913 Edition
Opal
O′pal
,Noun.
 [L. 
opalus
: cf. Gr. [GREEK], 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. 
☞ 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
. Webster 1828 Edition
Opal
O'PAL
,Noun.
  Opal is a subspecies of indivisible quartz.