GEMS AND PRECIOUS STONES. 15
was
found to be composed of carbon, with a trace of metallic impurity. It
may seem remarkable that such a beautiful object should be composed of
such common material as carbon, but if we come to consider of what
material other gems are composed, we find that the ruby, sapphire, and
other gem varieties of corundum, consist of alumina, which is certainly
not less common than carbon, as it forms the basis of all clay and
other like deposits, while the majority of the remaining gems consist
largely of silica of which the greater portion of rocks principally
consist.
Simple forms of the Cubical system.
The
diamond crystallizes in the cubical system, that is, it has three
imaginary axes all at right angles to each other, and all of the same
length. The octahedron is the primary form, and this may be
considered as composed of two four-sided pyramids placed base to base,
with the axes joining each of the solid angles (a solid angle is the
meeting of more than two faces.)
Placing
six square planes equal to the length of one of the axes upon the solid
angles of the octahedron, a six-sided figure is obtained, called a cube or hexahedron, having six equal faces, the axes being of the same length, and joining the faces of the cube in their centre.
If
twelve planes are placed upon the edges of the octahedron (an edge
being a line joining two faces), and extending these faces until they
meet, a twelve-sided figure is obtained, called a rhombic dodecahedron, having
twelve faces, and each face a rhombus. It will be easily understood
that the axes in this figure are all of the same length, and that it
therefore belongs to the cubical system. This figure is a common one
for garnets, all of which crystallize ill this system. These, the most
simple forms of the cubical system, are easily distinguished ; models
can be easily cut from some soft substance, a potato for instance. The
appearance is also shown in the diagram.
It
is possible to have a portion of each of these three forms developed in
the same crystal, but the axes will not vary in length. A crystal
having these faces developed might at first sight be considered a
complex form, but it is not so, little consideration being necessary to
identify the faces.
Instead
of entirely removing the form of the octahedron, as in the cube and the
rhombic dodecahedron, crystals are often found with other faces formed
upon the octahedron. One form having three faces thus developed is
called a triakis octahedron ; it has twenty-four small faces,
each being an isosceles triangle, and they form obtuse three-sided
pyramids over each face of the octahedron.
A figure having six small faces developed on each face of the octahedron is called a hexakis octahedron for that reason ; it has forty-eight faces, and is not an uncommon form of the diamond.
By placing obtuse four-sided pyramids upon each of the six faces of the cube a figure is obtained called a tetrakis hexahedron, or four-faced cube ; it is bounded by twenty-four equal isosceles triangles.
These are the principal hoiohedral forms of the cubical system (meaning
those which possess the highest degree of symmetry of which the system
admits), necessary to be known in the discrimination of gems, although
perhaps the icositetrahedron should not be omitted, as it is common in
garnet and leucite. In general appearance this form somewhat resembles
the triakis octahedron, but its faces are deltoid in shape, and each solid
Flickr
Stumbleupon
Scribd