There are all degrees of precious stones, from the valuable diamond and corundums to the humbler quartz, amethyst, and topaz.
It has been a mooted question as to the proper dividing line between stones that deserve the title “precious,” and those which should be placed in a so-called semi-precious or lower category. To draw such a line is hardly possible, as neither hardness, rareness, nor value would be a positive test—some of the hard stones, like zircon and almandines being less valuable than the softer opal, while the diamond, one of the most plentiful of precious stones, is at the same time, one of the most valuable.
Neither can price be taken as a complete test, because fashion makes a turquois, an opal, or an emerald much more valuable at one time than at another. All precious minerals used for ornamental purposes, from the diamond to quartz, or chalcedony, may properly be termed precious stones.
Physical Characters.
CRYSTALLIZATION.
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 lateral axes equal. |
| 3 Hexagonal or Rhombohedral | ||
| 4 Orthorhombic or Trimetric |  | having the axes unequal. |
| 5 Monoclinic or Oblique | ||
| 6 Triclinic or Anorthic |