364 A POPULAR TREATISE ON GEMS.
serpentine with actinolite, &c. That
variety which contains amianthus in a layer, is sometimes exceedingly
beautiful; and when polished has the appearance of satin spar.
MARBLE.
This
is a carbonate of lime, and a wide range of minerals belong to this
class, containing substances which are subservient to architectural
and ornamental purposes; the author intends, therefore, treating this
subject more extensively and giving it a wider range than other common
minerals, and to copy from the jury report of the London and New York Exhibitions.
The
primary form of calcareous spar is an obtuse rhom-bohedron, with a
great many secondary forms; has a hardness of 2.5 to 3.5; specific
gravity, 2.5 to 2.7; it has a vitreous lustre, also earthy; white or
grayish-white streak ; color usually white, with a great variety of
shades of gray, red, green, and yellow, also brown and black; it is
transparent and opaque, the transparent varieties exhibit double
refraction very distinctly ; fracture usually conchoidal, but obtained
with difficulty, when the specimen is crystalline. It is composed of
.lime and carbonic acid, the colored varieties often contain, in
addition, small portions of iron, silica, magnesia, alumina, and
bitumen, and acids produce a brisk effervescence; before the blowpipe
it is infusible,—it loses, however, its carbonic acid, gives out an
intense light, and ultimately is reduced to pure lime, or quicklime.
Calcareous
spar appears under a very great variety of forms and aspects; a great
many species have, therefore, been created by mineralogists.
Iceland spar was first applied to a transparent crystallized variety from Iceland, where it was found in a eavity
