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Attempts have been made, however, to decolorize diamonds in which the undesirable pigment is distributed through the substance of the whole stone. Probably the first to make experiments in this direction was the Emperor Rudolph II.; according to the report of his gem expert, Boetius de Boot, the Emperor was acquainted with a method by which every diamond could be decolorized, and rendered perfectly colorless. This important secret, however, died with its possessor. At a later date the Parisian jeweler, Barbot, claimed to be able, by the employment of chemical means and a high temperature, to decolorize green, red, and yellow stones, while dark yellow, brown, and black stones only slightly lost their color. Barbot's method was also preserved as a secret, so that it is impossible to put his assertion to the test; it is probable, however, that he did not possess the power to which he laid claim, in spite of the fact that he described himself on the title page of one of his books as "Inventeur du procede de decoloration du diamant." Our present knowledge of the constituents of diamond pigments makes us unwilling to believe too readily in the possibility of their complete destruction; at all events, no method is at present known which is effectual in all cases.

Although it is impossible actually to destroy the color of yellow diamonds, such as now so often come into the market, it is easy to disguise their yellow tint and make them appear colorless. This device has often been practiced for fraudulent purposes, and was successfully carried out in Paris a few years ago. The yellow stone is placed in a violet liquid, such as the dilute solution of potassium permanganate used for disinfecting purposes. On being taken out and allowed to dry the stone becomes coated with a thin film of the violet substance. The combined effect of yellow and violet colours in certain proportions is to produce in the eye the sensation of white light; hence, when the violet layer is of a certain thickness, the stone will have the appearance of being perfectly colorless. Should it still appear yellow after the first trial, it may be immersed a second or third time, while if the violet color is too deep, some of it may be washed off. Instead of a solution of potassium permanganate violet ink may be used for this purpose. A stone treated in this way will, of course, remain apparently colorless only so long as the violet coating remains intact; directly this is rubbed off, which readily happens, the yellow color of the stone becomes apparent. A more permanent coating is said, however to have been recently devised for this purpose. The methods by which yellow stones can be made to appear colorless are very old; ultramarine is supposed to have been used by the ancient inhabitants of India for this purpose.
According to the investigations of Petzholdt, which are confirmed by every diamond cutter, the color of diamond powder or dust varies from gray to black, and is darker the finer its state of division.


Many erroneous statements have been made with regard to the phosphorescence of diamond. Thus it has been stated that diamond phosphoresces in darkness after exposure to the direct rays of the sun, the phosphorescence being specially marked after exposure to blue light, and less so after exposure to red light. It is even said that after being screened by a board or paper from the direct rays of the sun, so that the stone is exposed to diffuse daylight only, it will be seen to glow brightly when placed in darkness.
Exact researches, however, have proved that only very few diamonds phosphoresce after exposure to sunlight, and that neither the direct rays of the sun nor intense artificial light cause phosphorescence in the majority of stones. Streeter reports that a yellow stone of 115 carats, after exposure to lime-light lit up a dark room; and Edwards describes a water-clear diamond of 92 carats, which after one hour's exposure to electric light emitted in a dark room a light which lasted for twenty minutes, and was so strong that a sheet of white paper placed near the stone could be distinctly seen. Of 150 diamonds of various forms, sizes and qualities examined by Kunz, only three showed the phenomenon of phosphorescence after exposure to the light of the electric arc.
Although exposure to light has in diamond so small an effect in exciting phosphorescence, rubbing the stone easily produces the phenomenon. Kunz observed that all the diamonds he examined, without exception, became self-luminous after being rubbed on wood, leather, woolen or other material. Some stones needed only to be drawn once across the substance, especially if it were of wool, to render them self-luminous; the most marked phosphorescence was, however, developed in stones by rubbing on wood against the grain. According to other statements, rubbing on metals (iron, steel, copper) is effective.
X-rays induce phosphorescence in most of diamonds, if not all. This property is used to separate diamonds from concentrate. The compressed air powered x-ray machines replaced old grease tables.
Whether diamonds are capable, as a rule, of giving out light after being raised to a temperature below red heat is doubtful, but some specimens, which are unaffected by sunlight and remain quite dark, are induced to glow by exposure to electric sparks. But here, as in all cases, the phosphorescence can be produced only when the stone has not previously been exposed to a strong heat.
The light given out by a phosphorescing diamond is in almost all cases feeble, and much less intense than that emitted by many other phosphorescent substances. The light, which is usually yellow in color, but may under certain circumstances be blue, green, or red, is strongest when induced by electric sparking. Remarkable observations have been made on the appearance of different faces of a phosphorescing crystal of diamond. Dessaignes (1809) stated that a diamond after exposure to the sun's rays emitted light from the cube faces, but not from the octahedral faces, which remained quite dark. Maskelyne described a diamond crystal, which emitted a beautiful apricot-colored light from the cube faces, a bright yellow from the faces of the rhombic dodecahedron, and a yellow light of another shade from the octahedral faces.
All these appearances are, as a rule, of brief duration. A case is, however, recorded of a diamond, which continued to phosphoresce for an hour after the removal of the exciting cause.
The phosphorescence of diamond is said to have been first observed in 1663 by the famous English physicist Robert Boyle.


A diamond, whether rough or cut, becomes positively electrified on rubbing; the charge so acquired is quickly lost, never being retained more than half an hour. In contrast to graphite, the other crystallized modification of carbon, which is a good conductor of electricity, the conductivity of diamond is so inappreciable that the stone ranks as a nonconductor.
Diamond being a good conductor of heat appears cold to the touch, and by this means can be distinguished from other substances, as already explained.


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March, 2011