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BOOK VI
115
that are not as hard as a diamond are engraved more easily with a fine iron wire that is first coated with oil and then with emery powder. They are cut more quickly by this method than with a diamond point since the engraver can use different wires, sharp and dull, large and small. It costs the engraver little to use emery: a considerable sum to use diamond. But this is enough concerning the relief engraving of gems.
Not only is a pleasing angular form given to transparent gems in order to increase their brilliancy but when they are set in rings it is customary to place bright thin foil of almost the same color beneath the stone and this adds its color to that of the stone. This foil is made in the following way. Gold, silver and copper are melted together into a bar. This is then drawn out into very thin sheets. These are held by pinchers over burning coals in such a way that they do not touch the coals yet absorb some of their heat. This heat stains them various colors depending upon whether gold predominates over silver and copper, silver over gold and copper, or copper over silver and gold. Yellow foil is placed under topazius; green under smaragdus and chrysoberyllus; blue under sapphirus^ and sapphire; red under carbunculus. If one suspects that a gem has been embellished and enlightened with foil and wishes to know the true color of the gem he must take the stone out of the ring and remove the coloring substance. Since fraud begins with a single act and once begun is hard to stop, I shall mention a few of the many ways in which gems are falsified as well as a few ways in which true gems can be distinguished from the false so that anyone may detect them and thus protect himself against fraud.
Glass, as I have said, is dyed many colors and may have the same color as smaragdus, turquois, amethyst, hyacinthvs, chrysolithus and topazius. This genus of artificial stones cannot be recognized by their appearance but can be detected by drawing a file across them. The glass, because it is soft and fragile, is scratched by the file while the true gem, being hard, is not affected, except topazius and smaragdus and even these stones are not scratched if they are Scythian or Egyptian. True gems may also be distinguished by touch since glass is warmer when compared with a gem. Glass is lighter than a gem. By these two methods true topazius and smaragdus can be distinguished from false. The eye may detect bubbles in the glass which sometimes shine like silver in the depth of the stone. The permanent brilliancy of the true gem is always appearent to the eye and is very soothing while any brilliancy of glass fades or dies before it ever reaches the eye. This can be readily observed if we examine the gems in early morning light or at night by the light of a lamp. Glass is usually rough on the surface.
Transparent gems are sometimes made from dyed quartz but this fraud is detected by using a file and by sharp eyesight. These false gems have
* Here sapphirus must refer to the modern sapphire and be synonymous with cyanus. Foil would add nothing to the color of lapis-lazuli.