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 |
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.
CLEAVAGE.
Many minerals can be separated readily in one direction by simply making a small indentation with a harder mineral, then introducing the blade of a knife into the scratch and striking it a sharp blow,—this separates the crystal. There are certain planes at right angles where the crystal can be separated; this property is called cleavage and the planes, cleavage planes.
In some minerals cleavage is difficult to produce, while in others such as mica and rock-salt, cleavage is highly perfect and the number of separations produced is only limited by the thickness of the blade used in separating the planes.
The property of cleavage is very useful and of great assistance to the lapidary, as it enables him to shape a diamond or other hard stone nearly to the size he desires, and at the same time to save the extra material cleaved off, which can be used for smaller gems, and which under other conditions would have to be ground away.
FRACTURE.
Fracture surfaces are the result of the breaking of a crystal otherwise than by cleaving, and in a different direction from the cleavage planes.
When the form of fracture is composed of concave and convex surfaces it is called conchoidal; when free from inequalities it is known as even or smooth, and when covered by small splinters, splintery or uneven.
