cat all gems except the diamond. For fuller description see Kraus and Slawson, pp. 102-112.
After the facets have been cut, the stone is then polished with tripoli or rouge on wooden cylinders or on canvas-covered wheels. The methods are also described in Quartz Family Minerals, by Dake, Flower and Wilson and Jewellery, Gem-cutting and Metalcraft by William T. Baxter, McGraw Hill Book Co., Inc. New York.
Engraving
It is a specialized branch of gem-cutting for the cutting of cameos, etc., requiring great skill. It is done by a small lathe into which suitable cutting tools are fitted. The stone is held against the rapidly revolving tool. Engraving and boring are done as home industries in Idar-Oberstein.
Size and Weight of Gems
The usual units for weighing gem-stones are: (i) carat, (ii) grain, (iii) pennyweight, and (iv) grain. The carat is the most used in the gem trade in the principal countries of the world. It is 200 milligrams or 2/10 of a gram. It is also known as the metric carat. Generally the value of a gem-stone per carat increases rapidly with weight. Formerly, before the introduction of the metric carat, the carat had different weights in different countries.
London—0-2053 grams. Madras—0-2073 grams. Florence—0-1972 grams. Amsterdam—0-2057 grams.
Now decimals of a carat are used instead of fractions. The term carat used for gems should not be confused with the carat used to denote the fineness of gold. Here a carat means 1/24 part and pure gold is 24 carats fine, i.e. 18 carat gold is 18/24 gold and 6/24 base metal.
Lapis lazuli, citrine, moonstone, amethyst, etc., are sold by the gram. Pennyweight (1-42 grams) may also be
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