JADE
Jade
is a term applied in general to a tough, fibrous mineral of a greenish
color having the composition of a pyroxene or amphibole. Until recently
the mineral was supposed to form a single species, but it is now known
that at least two species are grouped under this title. One of these is
a form of pyroxene, and is known as jadeite, the other is a form of
amphibole, and is named nephrite. Further, the term is often used to
include any tough green stone having a hardness between 6 and 7 and
taking a good polish, since such rocks or minerals are often carved by
people who use true jade. Jadeite is a mineral of definite composition,
it being a silicate of soda and alumina. The percentage composition of
pure jadeite is, silica 59.4 per cent, alumina 25.2 per cent, and soda
15.4 per cent. Its hardness is a little below that of quartz, or
between 6.5 and 7, but its extreme toughness makes it often seem harder
than this. It is a rather heavy mineral, its specific gravity being
3.35. In color it varies from nearly white to nearly emerald-green. The
white varieties sometimes contain spots of bright green, supposed to be
due to chromium. The mineral does not crystallize, but is known from
its optical properties to be either monoclinic or triclinic. It does
not occur transparent, but has a peculiar translucency or
subtranslucency not unlike that of horn or fine porcelain. It has a
fibrous to granular structure readily seen under the microscope, and a
splintery fracture. It is very tough. Jadeite fuses readily before the
blowpipe to a transparent, blebby glass, and colors the flame yellow,
thus differing from nephrite, which is almost infusible. The term
chloromelanite is applied to a dark green to black jadeite containing
considerable iron.
Objects
of jadeite carved in prehistoric times are found abundantly in Europe,
Asia, America, and Africa, but only a few of the original localities
whence it was obtained are now known. The most important locality known
at the present time is in Upper Burmah in the vicinity of Mogoung. The
jadeite occurs in boulders embedded in a reddish yellow clay in the
valleys of tributaries of the Dschindwin River. The boulders are mined
by digging shallow pits after the fashion of the Burmese miners, as
many as a thousand men often being employed in this work. The miners
break the boulders by heating, and when pieces of
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