Varieties of Opal
Besides the noble opal there are several other varieties.
Fire
opal, or girasol, is more transparent than the precious opal, softer,
and the prismatic colors are less distinct. The body color is yellow to
a bright brownish red, in which the prismatic colors float. It is found
in Mexico and the Faroe Islands.
Common
opal, or semi-opal, is very similar to the noble opal, except that the
play of prismatic colors is very indisÂtinct and the milky body lacks
brilliancy.
Hydrophane
is an opal which has lost brilliancy by the evaporation of its water.
It becomes temporarily translucent by soaking in water or alcohol. If
boiled in oil, it will retain its brilliancy for some time.
Cacholong is milky white and nearly opaque. It is similar to and often confounded with chalcedony.
Hyalite, a glassy variety, is valueless as a gem. It is found in the United States.
Menilite is a brown, opaque, and slaty variety.
Wood opal is wood petrified with a hydrated silica. The color is gray, brown, or black.
Other forms are simply silicas having some opalescent features insufficient to warrant attention as gems.
