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To M. Antony Jacques, a jeweller of Grenoble, France, is accredited the discovery of a new method of detecting counterfeit emeralds and garnets, a method that is simple and that can be applied by any person. Through two coloured glasses, placed across and upon one another, one blue and the other yellow, the stone in question is examined, the stone being placed directly against an electric lamp. The genuine emerald will appear to be of a violet colour, no matter whether it is a "scientific," a "reconstructed" gem, or an ordinary green doublet. The most convincing imitation will appear unchanged and the deception thus easily demonstrated. A genuine garnet similarly placed upon an electric lamp and looked a through pale-green glass will appear decolourised, while a counterfeit will remain a garnet colour. The author's experiments have demonstrated the efficiency and reliability of these tests.
Besides the complete imitation of gems there are partial sophistications in which considerable ingenuity and constructive ability are displayed by creating "doublets" and "triplets." The doublet is constructed with the table and crown of a genuine stone, usually off-coloured, cemented to a pavilion made of a paste having the approved colour, thus giving the valueless crown the appearance of a fine stone. The softness of its pavilion usually betrays the doublet. As a guard against this discovery the triplet was invented. This is a real gem, usually pale or off-coloured, with a thin layer of coloured glass at the girdle. The detection of this combination usually requires the magnifying glass and specific gravity tests; the glass usually betrays the deception, and if soaked in alcohol, carbon bisulphide, or ether, the fraud usually separates. Pearls are imitated by coating the inner surfaces of glass beads with a preparation made from fish-scales.
Substitution of other minerals for specific precious stones has not the shadow of justification that sometimes softens the annoyance of receiving, or being offered, "something just as good" in drugs, groceries, or dry goods. The substitutes sometimes offered or proposed for diamonds include white sapphires, zircon, quartz, and white topaz. Artifice is frequently employed to heighten or change the colour of a real gem by thermal or chemical treatment; thus heat may remove the colour or increase the brilliancy of topaz, sapphire, and other precious stones. Heat will change the colour of a wine-yellow Brazilian topaz to a rose-pink; the same influence may whiten and render more brilliant an off-coloured or spotted diamond. A high temperature will often alter and improve the colour of the cairngorm, citrine quartz, and other minor gems. Chemical solutions can be successfully applied to turquoise to deepen its colour and invest it with permanency; agates are commonly dyed, and by chemical aid colourless chalcedony is converted into an excellent imitation of the moss agate. An off-coloured diamond may be apparently changed to a stone of good water by a wash of aniline blue, but the effect is but temporary. Besides these, the interiors of settings may be backed, stained, or enamelled, usually entirely legitimate improvements. |
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Precious Stones Guide Vol 4
>> M. Antony Jacques and Historical Gem Counterfeit Detection
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