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Light.--When the most valuable precious stones, and the diamond particularly, are exposed for a certain time to the rays of the sun, and are then taken into darkness, they remain luminous, and exhibit the phenomenon of phosphorescence. This curious effect lasts for some time, but gradually becomes fainter and fainter, and finally disappears.
Heat.--The effects produced upon precious stones by heat are even more remarkable than those due to the action of light. Heat acts upon them in two very dissimilar ways. It modifies the elementary constitution of the stone by separating its molecules, but this in a manner altogether mechanical; or it produces in the stone a veritable chemical reaction. In the first case the modifications are temporary, and at length the objects return to their primitive condition; in the second case the effects produced are permanent.
As an example of the latter case, we may cite a practice whose origin is lost in antiquity, and which is still resorted to daily by lapidaries. It consists in submitting a coloured stone (diamond, topaz, &c.) to a temperature more or less elevated. Nearly always in these conditions the stone changes colour permanently.
A remarkable communication made to the Academy of Sciences will serve as an example of the first case.
"MM. Halphen have the honour to present to the Academy a diamond of the weight of 4 grammes (about 20 carats), presenting a phenomenon which has never been before observed, at least to their knowledge.
"This stone is, in its normal state, of a white colour, faintly tinged with brown. When it is subjected to the action of fire, it acquires a very clear rose-tint, which it retains for eight or ten days, and which it loses gradually, to return to its primitive normal colour.
"This change and return to the primitive state may be repeated indefinitely, for the diamond submitted to the Academy has been subjected five times to this test.
"The phenomenon in question arrested at first the attention of an observer, who was trying at random upon this diamond the prolonged action of fire. Experiments made since upon other diamonds have not produced the same result.
"This question of colouring diamonds has an importance which the Academy will easily appreciate, when it considers that the stone presented at this moment has, in its normal state, a value of 60,000 francs, while its price in the rose-coloured state, if the colour were permanent, would be from 150,000 to 200,000 francs." |
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Precious Stones Guide Vol 2
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