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An excerpt from A Textbook of Mineralogy by Georgius Agricola (1494 – 1555):

However, irrespective of the manner in which gems occur they are never as brilliant and transparent in the natural state as when polished. They are first polished with an earth called tripoli.2 This earth is sprinkled on a lead wheel which is rotated with the right hand while the gem is held firmly against it with the left. Whether the gem is polished or engraved it is always cemented, with a variety of pitch, to the end of a wooden spindle which is held in the hand. In order to give the gem a higher polish it is held tightly against a wooden wheel covered with the hide of an elk or some equally thick skin. Thus the old method of polishing gems on Naxian or Armenian whetstones has been changed. Artisans usually give an angular shape to certain massive gems before they are polished. Although the hexagonal smaragdus, carbunculns, sapphire and even the diamond have natural faces their brilliancy is enhanced by cutting new facets on them. The dull color of the hexagonal beryl of India is enlivened by the reflections of the angles. Many facets are added to European quartz and jaspis in order to make them more becoming as ornaments. Powdered emery is sprinkled on brass wheels when these are used instead of lead and first one part of a stone is held firmly aganist the wheel and then another part until it is faceted. All gems can be cut with emery powder except diamond which can only be cut with the diamond powder brought from India by the Lusitanians. Gems are also suitable for relief engraving and the finest figures are carved on them. Some gems can be engraved more easily than others, for example, carnelian, onyx, sardonyx, amethyst, jaspis, molochites and smoky quartz. Certain others stubbornly resist efforts to engrave them, for example, Indian diamond, Scythian and Egyptian smaragdus,’ sapphirus* sapphire and the carbuncidus which holds part of the wax in the signet. Archelaus writes that the Carthaginian garnet (carchedonius) can melt the wax from signets, even the deepest part. Although the diamond is the most difficult of all stones to engrave this is sometimes done to conceal a flaw in a prominent part but because of the hardness and the difficulty in cutting it, the stone is usually hollowed out with the sharp point of another diamond or with diamond fragments set in iron. This iron is set in a square hole in a brass shaft. A heavy cord goes around this shaft and down around a wheel. The engraver, turning the wheel with the right hand and at the same time rotating the shaft, applies the gem set in pitch to the diamond point with the left hand. The diamond which is to be engraved is cemented with pitch to the end of a wooden spindle. In this way figures are engraved on a gem.