This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
If characters or marks of any kind are drawn with an aluminum pencil on glass, porcelain, or any substance containing silex, the marks cannot be erased by rubbing, however energetic the friction, and even acids will not cause them to disappear entirely, unless the surface is entirely freed from greasy matter, which can be accomplished by rubbing with whiting and passing a moistened cloth over the surface at the time of writing. So, in order to distinguish the true diamond from the false, it is necessary only to wipe the stone carefully and trace a line on it with an aluminum pencil, and then rub it briskly with a moistened cloth. If the line continues visible, the stone is surely false. If, on the contrary, the stone is a true diamond, the line will disappear without leaving a trace, and without injury to the stone.
The common test for recognizing the diamond is the file, which does not cut it, though it readily attacks imitations. There are other stones not affected by the file, but they have characteristics of color and other effects by which they are readily distinguished.
This test should be confirmed by others. From the following the reader can select the most convenient:
A piece of glass on which the edge of a diamond is drawn, will be cut without much pressure; a slight blow is sufficient to separate the glass. An imitation may scratch the glass, but this will not be cut as with the diamond.
If a small drop of water is placed upon the face of a diamond and moved about by means of the point of a pin, it will preserve its globular form, provided the stone is clean and dry. If the attempt is made on glass, the drop will spread.
A diamond immersed in a glass of water will be distinctly visible, and will shine clearly through the liquid. The imitation stone will be confounded with the water and will be nearly invisible.
By looking through a diamond with a glass at a black point on a sheet of white paper, a single distinct point will be seen. Several points, or a foggy point will appear if the stone is spurious.
Hydrofluoric acid dissolves all imitations, but has no effect on true diamonds. This acid is kept in gutta-percha bottles.
For an eye practiced in comparisons it is not difficult to discern that the facets in the cut of a true diamond are not as regular as are those of the imitation; for in cutting and polishing the real stone an effort is made to preserve the original as much as possible, preferring some slight irregularities in the planes and edges to the loss in the weight, for we all know that diamonds are sold by weight. In an imitation, however, whether of paste or another less valuable stone, there is always an abundance of cheap material which may be cut away and thereby form a perfect-appearing stone.
Take a piece of a fabric, striped red and white, and draw the stone to be tested over the colors. If it is an imitation, the colors will be seen through it, while a diamond will not allow them to be seen.
A genuine diamond, rubbed on wood or metal, after having been previously-exposed to the light of the electric arc, becomes phosphorescent in darkness, which does not occur with imitations.
Heat the stone to be tested, after giving it a coating of borax, and let it fall into cold water. A diamond will undergo the test without the slightest damage; the glass will be broken in pieces.
Finally, try with the fingers to crush an imitation and a genuine diamond between two coins, and you will soon see the difference.