A first-class ruby has the color of the blood as it spirts from an artery. The deeper the hue of the emerald the more it is valued; it loses none of its brilliancy by artificial light. The pale rose topaz, the kind most esteemed, is artificially colored by heating it. If topaz or tourmaline be gently heated, it becomes electric and will attract a thread or suspended pith-ball. No imitation will do this. All real gems when rubbed will attract the pith-ball, and retain the power a long time; the pastes also become electric, but soon lose their attraction. Rub a glass tube with a piece of flannel and bring it near a suspended pith-ball; the latter will be strongly attracted and then repelled. Immediately rub a genuine diamond and bring it near the ball, and it will be attracted. A paste diamond thus rubbed would repel it. Finally: the breath remains much longer on the pastes than on real gems. The former also betray under a magnifying glass small air bubbles. Diamonds and other first-class stones are always cold to the touch. FALSE PEARLS.--These are glass beads coated with a mixture of three ounces of scales of the blay or bleak fish, half an ounce of fine glue, one ounce of white wax and one ounce of pulverized alabaster. Powdered opal is sometimes used; also the powdered pearl of the oyster and other shells soaked in vinegar, and made up with gum tragacanth. Artificial pearls are usually brittle, and do not weigh more than two-thirds as much as the genuine. FALSE CORALS.--These are made of resin and vermilion; or of marble powder made into a paste with varnish or soluble glass and a little isinglass, colored by Chinese vermilion, and then moulded. They are used for setting in cheap jewelry. The knife shows it to be too soft to be genuine. ARTIFICIAL GOLD.--The following oroid or imitation gold is sometimes sold for the genuine article which it closely resembles. Pure copper, 100 parts by weight, is melted in a crucible, and then 6 parts of magnesia, 3.6 of sal-ammoniac, 1.8 of quicklime and 9. of tartar are added separately and gradually in the form of powder. The whole is then stirred for about half an hour, and 17 parts of zinc or tin in small grains are thrown in and thoroughly mixed. The crucible is now covered and the mixture kept melted for half an hour longer, when it is skimmed and poured out. Any imitation of gold may be detected by its weight, which is not one-half of what it should be, and by its dissolving in nitric acid while pure gold is untouched. PRECIOUS STONES. ARRANGED ACCORDING TO COLOR AND IN ORDER OF HARDNESS. _Limpid._ Diamond, Sapphire, Topaz, Rock-Crystal. _Blue._ Sapphire, Topaz, Spinel, Aquamarine, Indicolite, Turquoise, Kyanite. _Green._ Oriental Emerald, Chrysoberyl, Emerald, Chrysoprase, Chrysolite, Amazon Stone, Malachite. _Yellow._ Diamond, Topaz, Fire-Opal. _Red._ Sapphire-Ruby, Spinel-Ruby, Brazilian-Topaz, Hyacinth, Carnelian, Rubellite, Garnet. _Violet._