Lava to Zircon

Lava can hardly be classed as a semi-precious stone, but it is and has been quite extensively utilised in jewelry, chiefly on account of sentimental association with, and as souvenirs of, volcanoes. Lava is the fusion of various mineral substances due to the heat and force of eruptions from the interior of the earth; it varies in structure and constituents, but the surface lava is usually massive with vesicular or porous marks; fracture, splintery and conchoidal; lustre, dull or glistening; it is opaque and of various colours and shades. Lava frequently contains crystals—feldspar, lenate, hornblende, garnet, and other minerals. Vesuvian lava of a blue tint resembles transparent enamel, and is mounted in brooches and rings; cameos and intaglios are sometimes cut on it.

Magnetite, or lodestone, possessing polarity, is used for charms, because of the mystical properties attributed to it.

Malachite is carbonate of copper of a bright green colour. When this copper ore occurs in conjunction with azurite, the companion minerals are cut together, with a pleasing effect.

Obsidian is compact volcanic glass, and is cut for gem purposes to a greater extent than some of the other semi-precious stones here referred to. Varieties are moldavite, or bottle stone, of a green colour; marekanite or mountain mahogany, a red or black and brown banded kind; and Iceland agate; pearlylite; and sphaerulite.

Phenacite, of gem quality, is transparent, colourless, and of a vitreous lustre. This brilliant mineral is harder, heavier, and more refractive than quartz, which it so closely resembles, so that it was not until 1833 that mineralogists differentiated between them. Its name, phenacite, is derived from the Greek word phenax, meaning a deceiver. Phenacite remotely resembles the diamond in its brilliancy and refractiveness. Some specimens exhibit pale-rose and wine-yellow colours.

Pyrite is a brass-yellow mineral of metallic lustre known to jewellers as sulphur-stone and technically as marcasite. It is a common mineral, and is so frequently mistaken by the uninformed for gold that it has earned the sobriquet "fool's gold." Pyrite is a sulphide of iron. Although so common as to have no intrinsic value, pyrite constantly remains in use in jewelry and is seen in rings, brooches, and scarf pins. In coal-mining regions it is sold as souvenirs, mounted in medium to low-priced settings.

Rutile is oxide of titanium, containing more or less iron. When sufficiently transparent and brilliant, its qualifications of hardness and adamantine lustre make it a very desirable gem stone. The colour of rutile is usually reddish brown, but some specimens when cut closely resemble the ruby. Rutile sometimes forms hair-like crystals penetrating quartz and other transparent minerals with a very curious and beautiful effect.

Satin Spar is a form of gypsum, white, with a delicately fibrous structure, which, when polished, exhibits a beautiful silky lustre and pearly opalescence. It appears in the jewelry trade sometimes in necklaces, charms, and pendants. A great trade in these articles is a constant source of revenue at Niagara Falls, tourists being ready purchasers on the assumption that the mineral is native to the region; gypsum is found at Niagara Falls but not of this variety; the raw material thus used is obtained in Wales.

Staurolite occasionally appears among the stones handled in the trade. Colour, reddish brown to brownish black; hardness, 7.5. The transparent kinds when cut resemble garnets. Because of their resemblance to a cross, the twinned forms are used to quite an extent as ornaments and charms; there is a tradition that they fell from heaven.

Titanite, or sphene, possesses an adamantine lustre, as does the diamond, and gems cut from this mineral are quite effective, but they lack the desired depth of colour and hardness to confer upon them a higher rank in the company of gems. The colour range of titanite is considerable, and transparent pieces, according to their colour, when cut, resemble topaz, garnet, chrysolite, and other stones. Sphene crystallises in the monoclinic system, the crystals frequently having the shape of a wedge; the name sphene is taken from the Greek sphen, meaning a wedge.

Zircon is alphabetically the omega of the semi-precious stones occasionally used, but, as it is probably used much more than any other mineral in this category, it is almost entitled to a chapter of its own. High specific gravity and an adamantine lustre are two marked characteristics of this mineral. Zircons are called "Matura diamonds," because of their abundance at Matura, Ceylon. Colourless or smoky zircons are called jargons or jargoons. Transparent zircons of a brownish, red-orange colour are called hyacinth or jacinth. Zircon is the heaviest gem mineral—more than four times the weight of water—its specific gravity being 4.2 to 4.86. Its hardness is 7 1/2. So high is its index of refraction—1.92—that it approaches the diamond in brilliancy when cut. Zircons of gem quality come mostly from Ceylon, where they are found in the form of rolled pebbles. Zircon is found in various American localities, but it is opaque.