COMMON OPAL.
This
mineral occurs massive and in rolled piece; also as stalactites; has a
conchoidal fracture; is transracent and semi-transparent; has a strong
vitreous and resinous lustre; its colors are. milky, yellow, reddish,
greenish-white, honey-yellow, wine-yellow, flesh, brick-red, and
olive-green ; sometimes dendritic (moss opal). Its specific gravity is
1.9 to 2.1.
The
wax or pitch opal is subordinate to this variety. It is found in the
same rocks as the precious opal, in Hungary ; in the hematite rocks of
Saxony; in the serpentine of Silesia; in cavities of trap and the
amygdaloid rocks of Iceland ; 'Faroe Islands ; and in the United States
(Pennsylvania and Connecticut).
It
is used for rings, pins, and cane-heads; but is, on the whole, not a
favorite among jewellers, and has no great value, because it is soft
and brittle; the paste, which may be made from white enamel, is
sometimes much prettier than the real stone.
HYDROPHANE.
The
name of this variety of opal has reference to its peculiar property of
becoming transparent and opalescent after immersion in water. The
ancients called this stone lapis mutabilis, and achates oculus mundi. It
is a common or precious opal, of porosis texture; adheres strongly to
the tongue; is translucent, and absorb water with avidity, giving off
at the same time air-bubbles; it thus assumes a high degree of
transparency, and sometimes the property of displaying the finest
prismatic colors, equal to the precious opal. This phenomenon tends
strongly to explain the display of the prismatic colors of the precious
opal; the more so, as the hydrophane loses this property on getting dry.
