Diamonds And Luxury

A complete account of precious stones ucla shows the completely developed crystal after being isolated from the mother-rock.

Some minerals, such as magnetite, are magnetic and respond to the influence of a magnet, being attracted by it. Magnetite has a black metallic lustre, and a certain titaniferous variety, namely iserine, takes when polished a very brilliant lustre like princess diamond earrings, and is sometimes used for ornamental purposes. The magnetic character of magnetite distinguishes it from other black stones, all of which are either non-magnetic or very feebly influenced by a magnet.

Many precious stones when exposed to certain external influences acquire a greater or less charge of electricity. They differ from each other in the length of time this charge can be retained, some retaining it for a considerable time, others for a less time, perhaps only a few minutes.

Different minerals differ very considerably in their conductivity for heat, and this character may serve in some cases to distinguish minerals similar in appearance from each other. The majority of precious stones are good conductors of heat, and on this account they are cold to the touch, since the heat of the hand is quickly conducted away. Glass is a somewhat poorer conductor and hence a glass imitation is not so cold to the touch as a genuine stone, since the warmth of the hand is not so quickly conducted away.

In this section we shall consider certain optical peculiarities and colour effects of a special and more or less abnormal kind ; these features are not shown by every specimen of a particular mineral species, but only by isolated examples. These appearances are governed by the ordinary laws of reflection and  refraction of light, and are due to the the material; they consist of an unknown substance which is very feebly refracting, but it is possible, however, that some are mere vacuities.

An important optical property of many precious stones is that known as dichroism or pleochroism. A stone possessing this property, when observed in different directions will show different colours or shades of colour which may resemble each other more or less closely, or may, on the other hand, differ considerably. A mineral sometimes used as a cut stone, and known as " water-sapphire,-" exhibits this phenomenon to such a marked degree that it has received the name dichroite, although at the present time it is usually known to miner¬alogists as cordierite.

An important character of precious stones not yet touched upon is colour. The beauty of opaque and lustreless stones, such as the turquoise, depends wholly upon this character. Every shade of colour is represented among minerals used as precious stones and for ornamental purposes. As has already been mentioned, stones which are perfectly colourless and also perfectly transparent are described as being water-clear, or of the first water.

The fact that doubly refracting crystals are singly refracting along their optic axes must not be forgotten in making observations with the polariscope. A doubly refracting stone, placed in the instrument so that its optic axis coincides with the line, of vision, will behave as if it were singly refracting, and will remain dark during a complete rotation of the carrier. A single observation of this kind is therefore not sufficient to prove the singly refracting character of the stone.

Hitherto we have considered only those substances in which a single refracted ray corresponds to a single incident ray. There are, however, many bodies, including many precious stones, which have the property of splitting a single incident ray of light into two refracted rays which are propagated in their substance along paths differing slightly in direction.

The refraction of light, and the phenomena connected with it, have an important bearing on the study of precious stones.
We have already noticed that of the light which falls upon a transparent body, such as a precious stone, a portion is reflected at the surface, while another portion enters its substance and is propagated in straight lines through it. When the incident ray of light