Precious stones are found either in crystallized or amorphous
conditions. The forms of crystallization are:
1 Isometric or Cubic; having the axes equal.
2 Tetragonal or Pyramidal } having only the
3 Hexagonal or Rhombohedral } lateral axes equal.
4 Orthorhombic or Trimetric }
5 Monoclinic or Oblique } having the axes
6 Triclinic or Anorthic } unequal.
Most precious stones crystallize, but the specimens that have the
crystallization clearly defined are seldom found. The amorphous
condition includes the turquois, opal, and obsidian, which minerals are
found in masses or veins surrounded by a matrix.