Part 2

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 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.

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.

OPTICAL PROPERTIES.

REFRACTION.

When a ray of light passes from one medium to another, or from the air to a crystal it is bent or refracted; this is called single refraction and takes place in the diamond, spinel, and garnet.

Most of the other transparent precious stones possess double refraction—that is, the ray of light enters the crystal and divides into two parts, one following the ordinary laws of refraction, while the other part or extraordinary ray does not obey the usual law.

There are precise methods for measuring the indices of refraction, but they are not applicable to polished gem stones.

POLARIZATION OF LIGHT.

Polarization is a peculiar modification which, under certain conditions, a ray of light undergoes. This property is easier to observe than double refraction.

If from a transparent prism of tourmaline two thin plates are cut, parallel to its axis, they will transmit light when they are placed above each other with the chief axis of each in the same direction.

When one of the plates is turned at right angles to the other, no light, or but very little, is transmitted, so that the plates appear black.

In passing through the first slip, the rays of light have acquired a peculiar property, which renders them incapable of being transmitted through the second, except when the two are held in a parallel position, and the rays are then said to be polarized.

In some doubly refracting crystals the two oppositely polarized beams are of different colors, so upon double refraction and polarization depends the property of many gems which is called pleiochroism.

PLEIOCHROISM.

The dichroiscope is a handy little optical instrument, that will readily serve to distinguish the diamond, spinel, or garnet (all singly refracting minerals) from the ruby, beryl, or any of the doubly refracting stones. This instrument consists of a cleavage rhombohedron of Iceland spar, fastened in a brass tube about 2½ inches long, and ¾ of an inch in diameter. A sliding cap at one end has a perforation ⅛ of an inch square, and at the other end is a lens which will show a distinct image of the square opening when the cap is pulled out about ¼ of an inch.

The pleiochroism of many stones can be determined at a glance with the dichroiscope.

When a stone is examined by means of the dichroiscope, it will show two images of the same hue, or of different hues, these square images (fig. 1, A) forming a right angle and being more distinct when viewed from one part of the stone than from another.