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 the stone, diaphaneity, as it is called; it may produce single or double refraction, or polarisation; if reflected, it may produce lustre or colour; or it may produce phosphorescence; so that light may be :
(2) reflected; or produce
The second physical property of light is seen in those stones which owe their beauty or value to Reflection: this again may be dependent on Lustre, or Colour.
This is an important characteristic due to reflection, and of which there are six varieties:
- (α) adamantine
- (β) pearly;
- (γ) silky;
- (δ) resinous;
- (ε) vitreous;
- (ζ) metallic.
These may be described:
(α) Adamantine, or the peculiar lustre of the diamond, so called from the lustre of adamantine spar, which is a form of corundum (as is emery) with a diamond-like lustre, the hard powder of which is used in polishing diamonds.
It is almost pure anhydrous alumina (Al2O3) and is, roughly, four times as heavy as water. The lustre of this is the true "adamantine," or diamond, brilliancy, and the other and impure divisions of this particular lustre are: splendent, when objects are reflected perfectly, but of a lower scale of perfection than the true "adamantine" standard, which is absolutely flawless.
When still lower, and the reflection, though maybe fairly good, is somewhat "fuzzy," or is confused or out of focus, it is then merely shining; when still less distinct, and no trace of actual reflection is possible (by which is meant that no object can be reproduced in any way to define it, as it could be defined in the reflection from still water or the surface of a mirror, even though imperfectly) the stone is then said to glint or glisten.
When too low in the scale even to glisten, merely showing a feeble lustre now and again as the light is reflected from its surface in points which vary with the angle of light, the stone is then said to be glimmering.
Below this, the definitions of lustre do not go, as such stones are said to be lustreless.
(β) Pearly, as its name implies, is the lustre of a pearl.
(γ) Silky, possessing the sheen of silk, hence its name.
(δ) Resinous, also explanatory in its name; amber and the like come in this variety.
This also explains itself, being of the lustre of glass, quartz, etc.; some experts subdividing this for greater defining accuracy into the "sub-vitreous" or lower type, for all but perfect specimens.
(ζ) Metallic or Sub-metallic.
The former when the lustre is perfect as in gold; the latter when the stones possess the less true lustre of copper.
Colour is an effect entirely dependent upon light, for in the total absence of light, such as in black darkness, objects are altogether invisible to the normal human eye.
In daylight, also, certain objects reflect so few vibrations of light, or none, that they appear grey, black, or jet-black; whilst those which reflect all the rays of which light is composed, and in the same number of vibrations, appear white.
Between these two extremes of none and all we find a wonderful play and variety of colour, as some gems allow the red rays only to pass and therefore appear red; others allow the blue rays only and these appear blue, and so on, through all the shades, combinations and varieties of the colours of which light is composed, as revealed by the prism. But this is so important a matter that it demands a chapter to itself.
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