A few minerals give out light when rubbed. In the year 1663 the Hon. Robert Boyle read a paper before the Royal Society, in which he described several experiments made with a diamond which markedly showed tribo-luminescence. As specimens of tribo-luminescent bodies I may instance sphalerite (sulphide of zinc), and an artificial sphalerite, which is even more responsive to friction than the native sulphide.[6]
Mrs. Kunz, wife of the well-known New York mineralogist, possesses, perhaps, the most remarkable of all phosphorescing diamonds. This prodigy diamond will phosphoresce in the dark for some minutes after being exposed to a small pocket electric light, and if rubbed on a piece of cloth a long streak of phosphorescence appears.
On passing a ray of light through a diamond and examining it in a spectroscope, Walter has found in all colourless brilliants of over 1 carat in weight an absorption band at wave-length 4155 (violet). He ascribes this band to an impurity and suggests it may possibly be due to samarium. Three other fainter lines were detected in the ultra-violet by means of photography.
But it is not the hardness of the diamond so much as its optical qualities that make it so highly prized. It is one of the most refracting substances in nature, and it also has the highest reflecting properties. In the cutting of diamonds advantage is taken of these qualities. When cut as a brilliant the facets on the lower side are inclined so that light falls on them at an angle of 24° 13´, at which angle all the incident light is totally reflected. A well-cut brilliant should appear opaque by transmitted light except at a small spot in the middle where the table and culet are opposite. All the light falling on the front of the stone is reflected from the facets, and the light passing into the diamond is reflected from the interior surfaces and refracted into colours when it passes out into the air, giving rise to the lightnings, the effulgence, and coruscations for which the diamond is supreme above all other gems.
The following table gives the refractive indices of diamonds and other bodies:
Chromate of lead 2·50-2·97 Diamond 2·47-2·75 Phosphorus 2·22 Sulphur 2·12 Ruby 1·78 Thallium glass 1·75 Iceland spar 1·65 Topaz 1·61 Beryl 1·60 Emerald 1·59 Flint glass 1·58 Quartz 1·55 Canada balsam 1·53 Crown glass 1·53 Fluor-spar 1·44 Ice 1·31