|
PRECIOUS STONES
|
13
|
||
|
|
|||
|
or
trimetric, monoclinic, and triclinic, of which are the alexandrite,
andalusite, cat's-eye, chrysoberyl, hiddenite, labrador-ite, malachite,
moonstone, olivine, sphene, and sunstone.
These
forms of crystallization are not often clearly defined, and are
generally rendered difficult to recognize in the stone as it is found,
by the modifications occasioned by the conditions surrounding it during
existence. The natural form of the diamond, for instance, is that of
two four-sided pyramids, united at their bases, or octahedron, but
there are many outward variations. It is also commonly found as a
rhombic dodecahedron, bounded by twelve lozenge-shaped faces or rhombs.
Sometimes it is found as a six-faced octahedron, and as the faces are
generally more or less curved, the forty-eight faces make it almost
spherical. It has been found quite globular in form, upon the surface
of which not the slightest appearance of a natural facet could be
discovered. So also with the ruby: although it is naturally a
six-sided prism, it is usually found as a rolled fragment, become so by
the processes which released and transported it from the matrix in
which it was born. Though harder than the surrounding elements through
which it ground its way to discovery during the centuries of its
existence, like all other things, the ruby seldom escapes the hand of
time scatheless; its own angles become rounded, its rough places are
made smooth, the outward features of its crystallization are worn away,
and the crystal emerges as a rolled pebble, worn by softer opponents,
as a rock is worn away by water.
The amorphous are those found in veins within a matrix. Of these are the opal, turquoise, and obsidian.
Many
precious stones have a grain or cleavage, along the lines of which they
can be split as wood is; and, as with woods, the cleavage is much more
perfect in some than in others. It is for this reason that hard stones
sometimes break so easily.
Except in a few cases, the fracture, or break across the
|
|||
|
|
|||