22 A PRACTICAL TREATISE ON GEMS.
nate
faces or groups of faces of a holohedric crystal increase
symmetrically, so as to obliterate the other faces. Thus, if four
alternate faces of the octohedron increase so as to obliterate the
other four, a tetrahedron with half the number of faces is formed.
I. The first, or Tesseral System, named from tessera, a
cube, which is one of the most frequent varieties, is characterized by
three equal axes intersecting each other at right angles. Properly
speaking, this system has no chief axis, as any one of them may be so
named, and placed upright in drawing and describing the crystals. Of
these there are thirteen varieties, which are thus classed and named
from the number of their faces:
1. One Tetrahedron, or form with four faces.
2. One Hexahedron, with six faces.
3. One Octahedron, with eight faces.
4. Four Dodecahedrons, with twelve faces.
5. Five Icositetrahedrons, with twenty-four faces.
6. One Tetracontaoctahedron, with forty-eight faces.
The
dodecahedrons are further distinguished, according to the form of their
faces, into rhombic, trigonal, deltoid, and pentagonal dodecahedrons;
and some of the icositetrahedrons have also received peculiar names.
