Conversion of Diamond into Graphite
Although we cannot convert graphite into diamond, we can change the diamond into graphite. A clear crystal of diamond is placed between two carbon poles, and the poles with intervening diamond are brought together and an arc formed between. The temperature of the diamond rapidly rises, and when it approaches 3600° C., the vaporising point of carbon, it breaks down, swells, and changes into black and valueless graphite.
Tribo-Luminescence
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.
Absorption Spectrum of Diamond
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.
Refractivity
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.