Crystal and stones » cubic system

Lapis-Lazuli

precious stones — Wed, 16 Sep 2009 16:16:15 +0000

The lapis-lazuli, sometimes called “azure stone,” is almost always blue, though often containing streaks of white and gold colour, the latter of which are due to the presence of minute specks or veins of iron pyrites, the former and colourless streaks being due to free lime, calcite, and other substances which have become more or [...]

Silicates

precious stones — Wed, 16 Sep 2009 16:14:00 +0000

The chief of these are the garnets, crystallising in the cubic system, and anhydrous.

The garnet is usually in the form of a rhombic dodecahedron, or as a trisoctahedron (called also sometimes an icosatetrahedron), or a mixture of the two, though the stones appear in other cubic forms.

In hardness they vary from [...]

Pleochroism and colour of minerals

precious stones — Wed, 16 Sep 2009 13:13:11 +0000

Color is one of the most wonderful effects in nature. It is an attribute of light and is not a part of the object which appears to be coloured; though all objects, by their chemical or physical composition, determine the number and variety of vibrations passed on or returned to the eye, thus fixing their [...]

Light and diaphaneity

precious stones — Wed, 16 Sep 2009 13:04:33 +0000

Probably the most important of the many important physical properties possessed by precious stones are those of light and its effects, for to these all known gems owe their beauty, if not actual fascination.

When light strikes a cut or polished stone, one or more of the following effects are observed:–it may be transmitted through [...]