The Opal 117
manganese
oxide, and is to opal as moss agate is to quartz. A freakish variety of
opal is tabasheer, a silica deposited within the joints of bamboo; it
is absorbent, and, like hydro-phane, becomes transparent when immersed
in water.
As
a mineral, opal is quite common, so that an amateur's collection of
minerals can include specimens to represent opal—some of them very
beautiful, too—at small cost, or for the effort of prospecting, in many
localities. The varieties of opal are many, and the frequent
inclusion of foreign matter invests it with a wonderful variety of
colours. The silica deposited by nearly all natural hot waters is
opalescent. The Yellowstone Park geysers shoot up around cones of opal
raised by the constant accretions of silica deposited by the passing
hot waters, which fall into opal basins created in the same way. This
variety of opal is termed geyserite. There is a wide gulf in values
between precious or noble opal—the gem stone quality—and opal in
general.
Opal
is generally found filling seams, cavities, and fissures in igneous
rocks, also embedded in limestone and argillaceous beds.
Opals of a quality fit for use as ornamental
