: 5. Zircon or Hyacinth. Its fundamental form is an isosceles 4-sided pyramid ; and the secondary forms have all a pyra midal character. Fracture, conchoidal, uneven ; lustre, more or less perfectly adamantine ; colors, red, brown, yellow, gray, green, white ; which, with the ex ception of some red tints, are not bright. Hardness, 7'5. Specific gravity, 4•5. 'Zir con and hyacinth consist, according to Klaproth, of almost exactly the same con stituents ; namely, zirconia, 70, silica, 25 ; oxyde of iron, 5. In the white zir conia there is less iron and more silica. Before the blowpipe the hyacinth loses its color, but does not melt. The brighter zircons are often worked up into a brilliant form, for ornamenting watch cases. As a gem, hyacinth has no high value. It has been often confounded with other stones, but its very great specific gravity makes it to be readily recognized.
6. Topaz. The fundamental form is a scalene 4-sided pyramid ibut the second ary forms have a prismatic character ; and are frequently observed in oblique 4-sided prisms, acuminated by 4 planes. The lateral planes of the prism are longitudin ally striated. Fracture, conchoidal, un even; lustre, vitreous ; colors, white, yel low, green, blue ; generally of pale shades. Hardness, 8; specific gravity, 3.5. Pris matic topaz consists, according to Berze lius, of alumina, 57.45; silica, 34.24; flu oric acid, 7•75. In a strong heat the faces of crystallization, but not those of cleav age, are covered with small blisters, which however immediately crack. With borax, it melts slowly into a transparent glass. Its powder colors the tincture of violets green. Those crystals which possess dif ferent faces of crystallization on opposite ends, acquire the opposite electricities on being heated. By friction, it acquires po sitive electricity.
Most perfect crystals of topaz have been found in Siberia, of green, blue, and white colors, along with beryl, in the Uralian and Altai mountains, as also in Rams chatka ; in Brazil, where they generally occur in loose crystals, and pebble forms of bright yellow colors ; and in Mucla, in Asia Minor, in pale straw-yellow regular crystals. They are also met with in the granitic detritus of Cairngorm, in Aber. deenshire. The blue varieties are absurd. ly called oriental aquamarine, by lapida ries. If exposed to heat, the Saxon topaz loses its color and becomes white ; the deep yellow Brazilian varieties assume a pale pink hue ; and are then sometimes mistaken for spindle, to which, however, they are somewhat inferior in hardness. Topaz is also distinguishable by its double refractive property. Tavernier mentions a topaz, in the possession of the Great Mogul, which weighed 157 carats, and oost £27,000 sterling. There is a speci men in the museum of natural history at Paris which weighs 4 ounces 2 grs.
Topazes are not scarce enough to be much valued by the lapidary.
7. Emerald and Beryl are describe6 in their alphabetical places. Emerald IcA es its lustre by candlelight; but as it ap pears to most advantage when in the com pany of diamonds, it is frequently sur rounded with brilliants, and occasionally with pearls. Beryl is the aquamarine of the jewellers, and has very little estima tion among lapidaries.
8. Garnet. See this stone in its alpha betical place.
9. Chrysolite, called Peridot, by HMV; probably the topaz of the ancients, as our topaz was their Chrysolite. It is the soft est of the precious stones, being scratched by quartz and the file. It refracts double.
10. Quartz, including, as sob-species, Amethyst, Rock - crystal, Bose - quartz, Prase, or Chrysoprase, and several vari eties of calcedony, as Cat's-eye, Plasma, Chtroprase, Onyx, &c. Lus tre, vitreous, inclining sometimes to re sinous • colors, very various ; fracture, conehoidal ; hardness, 7; specific gravity, 2•99.
11. Opal, or uncleavable quartz. Frac ture, conchoidal ; lustre, vitreous or re sinous ; colors, white, yellow, red, brown, green, gray. Lively play of light ; hard ness, 5'5 to 6'5 ; specific gravity, 2.091. It occurs in small kidney-shaped and stal Retitle shapes, and large tuberose concre tions. The Phenomena of the play of co lors in precious opal has not been satisfac torily explained. It seems to be connect ed with the regular structure of the min eral. Hydrophane, or oculis mundi, is a variety of opal without transparency, but acquiring it when immersed inwater, or in any transparent fluid. Precious opal was found by Klaproth to consist of silica, 90; water, 10; which is a very curious combination. Hungary has been long the only locality of precious opal, where it cc curs near Caschan, along with common and semi-opal, in a kind of porphyry. Fine varieties have, however, been lately discovered in the Faroe islands ; and most beautiful ones, sometimes quite transpa rent, near Gracias a Dios, in the province of Honduras, America. The red and yel low bright colored varieties of fire-opal are found near Zi inapan, in Mexico. Pre cious opal, when fashioned for a gem, is generally cut with a convex surface ; and if large, pure, and exhibiting a bright play of colors, is of considerable value. In modem times, fine opals of moderate bulk have been freqfiently sold at the price of diamonds of equal size : the Turks being particularly fond of them. The estima tion in which opal was held by the an cients is hardly credible. They called it Paideros, or Child beautiful as love. No nins, the Roman senator, preferred ban ishment to parting with his favorite opal, which was coveted by Mark Antony. Opal which appears quite red when held against the light, is called girasol by the French ; a name also given to the sapphire or corundum asterias or star-stone.
12. Tarquoi.e or Calcite. Mineral tar quois occurs massive ; fine-grained, im palpable ; fracture, conehoidal ; color, be tween a blue and a green, soft, and rather bright ; opaqe; hardness, 6; speo. gray., 2.83 to 3.0. Its constituents are alumina, 73; oxyde of copper, 4.5; oxyde of iron, 4 ; water, 13 ; according to Dr. John. But by Berzelins, it consists of phosphate of alumina and lime, silica, oxydes of cop per, and iron, with a little water. It has been found only in the neighborhood of Nichabour in the Khorassan, in Persia; and is very highly prized as an ornamental stone in that country. There is a totally different kind of turqnois, called bane Ni guel?, which seems to be phosphate of lime colored with oxyde of copper. When the oriental stone is cut and polished, it forms a pleasing gem of inferior value. Malachite, or mountain green, a compact carbonate of copper, has been substituted sometimes for tnrquois, but their shades are different. Malachite yields a green streak, and turqnois a white one.
13. Lapis lazuli is of little value, on ac count of its softness. (See ULTRA MA RINE.)