Diamond dust, black diamonds, and hard metals may be used to shape, engrave and polish the other stones. A list of some of the stones frequently used in ornaments and jewels would rank them, for hardness, in the following order: diamond 10 sapphire 9 ruby 8.8 chrysoberyl 8.5 spinel 8 topaz 8 aquamarine 8 emerald 7.8 zircon 7.8 tourmaline 7.5 amethyst 7 bloodstone 7 chalcedony 7 onyx 7 jade 6.5 peridot 6.3 moonstone 6.3 turquoise 6 opal 6 lapis lazuli 5.2 pearl 4 malachite 3.5 coral 3.5 amber 2.5 jet 2.5 _Qualities of a Stone_ The qualities that determine the value of a stone are difficult to specify. Hardness, size, weight and shape are obvious elements. Lustre and the powers of reflecting and refracting light clearly contribute to the value. The manner in which a stone is cut may add to its value, either because of the light effects or because of the interesting shape. One might expect perfection, freedom from flaw, to be important, and indeed in the diamond this is so. The most common flaw in the diamond, by the way, is not a crack but a speck or tiny specks of carbon remaining between the crystals, the diamond being a crystallized form of carbon. In other stones—as we have observed of the star ruby, the star sapphire and the cat’s eye—a physical flaw may result in a greater aesthetic desirability. Other special features may enhance the value of a particular stone; a recently discovered ruby is the only known example of a double star, with not six but twelve rays. The history and associations of a gem or jewel, dramatic or sentimental, storied or personal, may be what makes its possession desirable. _Measurement_ One seldom speaks of the size of a precious stone; other things being equal, its value is estimated by its weight. The unit of weight, in measuring precious stones, is the carat. As the word carat comes from the Arabic, meaning the nut or bean of the carob tree, it was evidently in olden times a rather imprecise measure. It has now been made definite as two-tenths of a gram (1c. = 0.2 gr.). It takes 141¾ carats to make an ounce, and therefore 2,268 carats to make a pound. Smaller diamonds are measured by points; one hundred points equal one carat.