This stone, sometimes called titanite from its chemical compo­sition (a silicate and titanate of calcium), varies in color, but only the transparent stones are cut and used in jewelry. Even so, it is seldom seen in mounted forms, for its softness mitigates against its more general use.

In color, it is generally green, yellow, or brown, the greens being more common, and they somewhat resemble the green beryl and chrysoberyl. Transparent stones show very strong dispersion and dichroism, two marked characteristics of this stone. The "fire" of a well cut, good quality sphene is remarkable, and it excels the diamond in this respect. In fact, with its brilliance, adamantine luster, and high refractive indices, it may be sometimes confused with a colored diamond, but it is very much softer. The refrac­tive indices are 1.90-1.98, specific gravity 3.4 to 3.6, but the hard­ness is only 5 1/2. The faces and edges of a cut stone are soon worn, if used in jewelry, and they need frequent repolishing to restore brilliance to the specimen.

The different colored varieties of this stone, which are known under different names, are unfamiliar to the general public. They are seen mostly in the U.S.A., where they are very popular. Cut stones are mounted in attractive jewelry, although good speci­mens are rare and realize high prices.

The general term spodumene, or spodumen, comprises various colored transparent stones of a like chemical composition. The chief is kunzite, a very beautiful limpid lilac-red or pink stone at its best, although many stones possess only a delicate, faint tinge of color. Hiddenite is the clear, emerald green variety, and there is a yellow spodumene of a lemon yellow shade.

All are very at­tractive and most suitable as jewel stones, but their comparative softness and easy cleavage are disadvantages. They are easily frac­tured, and great care must be taken in wearing and handling them. The easy cleavage is due to their perfect lamellar structure.

Crystals, often large, are usually prismatic in form, and they belong to the monoclinic system. Chemically, spodumene is a member of the pyroxene group of minerals and consists of a silicate of lithium and aluminum; it is one of the few minerals which contains the rare element lithium. Dichroism is very marked in all the varieties. Hardness varies from 6 1/2 to 7, specific gravity is 3.20, and refractive indices 1.66-1.67. Stones are faceted for use in jewelry, and they show a good luster. Kunzite is often cut very deeply, or "thick," as it is known in the trade. This deep cutting, by which the depth of the stone is increased in proportion to its width, is also seen in most Siberian amethysts and native cut sapphires, the object being to accentuate the color.

We generally think of garnet as being a dark red transparent stone, but the term really includes a number of minerals, only some of which are used in jewelry. Although they have the same crystalline habit—all fall within the cubic system—they vary slightly in their physical properties. Some are too dull and not sufficiently transparent to be attractive as gem stones, and al­though many different types may be seen in mounted jewelry, the most likely to be encountered are pyrope, hessonite, and alman-dine garnets. As there is no strict line of demarcation between the different species, it is often difficult to assign certain specimens to a particular class.

But all garnets consist of a double silicate, one of the metals being calcium, iron, magnesium, or manganese, the other being either aluminum, iron, or chromium. They may thus be divided approximately into six sub-species:

1.         Magnesium aluminum silicate garnet,

2.         Calcium aluminum silicate garnet,

3.         Iron aluminum silicate garnet,

4.         Iron calcium silicate garnet,

5.         Manganese aluminum silicate garnet,

6.         Calcium chromium silicate garnet.

The amount of silica present in all these varieties varies from 35 to 40 per cent. The common forms which the crystal takes is the rhombic dodecahedron and the icositetrahedron, sometimes singly and sometimes in combination, but they always fall into the cubic system of crystallography. Crystal shapes are often perfect, but when found in gem gravels, they are generally rounded. The faces are worn and they resemble round, water-worn pebbles. Twin crystals are also frequently found. 

Garnets are very widely distributed, and they occur in schists, gneisses, limestones, and serpentine. All are singly refractive, with no cleavage. Before the blow-pipe, most garnets fuse with ease. Specific gravities, refractive indices, and colors vary with the chemical constituents. The commonest variety is the pyrope gar­net, which has a hardness of 714, specific gravity 3.7 to 3.8, and a refractive index of 1.75. It is the most popular of garnets and may be seen in a variety of cheaper jewelry, particularly where clusters of small red stones are used.

At times, there is a fashion for red stones in jewelry, and it is then that pyrope garnets are used in quantity. Since they are cheap and attractive, brooches and rings which demand a number of small stones set in various forms make use of this stone, and the so-called Bohemian jewelry is universally known.

Pyrope is known under different names, such as Cape ruby, Arizona ruby, and Bohemian garnet, according to the district in which it is found. The color ranges from a fine deep red to a ruby red, but a yellowish or blackish tinge sometimes spoils the pure red. This is a magnesium aluminum silicate garnet, transparent and usu­ally without flaws. Large sizes are uncommon.

This gem stone has become much better known during the last few years since the blue and white varieties have been fairly ex­tensively used in jewelry. Actually, zircon is found in nearly every shade, green, blue, pale yellow, brown, orange, red, reddish-brown, and white, although violet colors are rare.

But not all the colors one sees are original; most of the blues and whites have been artificially treated and they were most likely brown when mined. Dark greens and yellowish-browns are the most common in nature, while the deep, golden yellow stones are rare and therefore reach a fairly high price. The white and pale yellow stones, sometimes known as jargoons, are the product of heating certain yellow and brown stones, as are also the bright blues, sometimes called starlites. The red and reddish-brown stones are occasionally known by the name of hyacinths, or per­haps more commonly, jacinths.

Siam and the adjacent French Indo-China provide most of the zircons we see, and it is here that the stones are treated, as well as cut and polished, before being exported. Suitable brownish ma­terial is selected and this is heated; if successful, either white or blue stones are the result. But in course of time, most of these change, the blues becoming paler and the whites showing a tinge of yellow or brown. Individual stones are affected differently by heat treatment, and some of the blue stones which have paled in course of time may have their color restored by reheating.

Like spinel, peridot is one of a large group of minerals, only a few of which are suitable for jewelry. These, known as olivines and chrysolites, are both transparent and of a greenish color. In jewelry, they are often used where a number of smaller green stones are required, for example, in clusters or in collet necklaces. The majority of cut stones that one sees is small, and large speci­mens are not common.

Gem material occurs as prismatic crystals, which are generally distorted or irregular in shape. They crystallize in the rhombic system and chemically are a silicate of magnesium and iron. Olivines are the darker green stones; the paler varieties, varying from a yellowish green to a bottle green in color, are called chrysolites, but the line of demarcation is rather elastic. Both names are frequently misapplied, the green garnet (deman-toid) very often being called olivine, while the green corundums and tourmalines are sometimes known as chrysolites. Peridot has a specific gravity of about 3.40, refractive indices, 1.65-1.69, and hardness of 6 1/2.

The hardness is sufficiently low on the scale to render this stone unsuitable for wear in rings, but owing to its attractive deep color, it is often used in this capacity. It is more suitable as a pendant stone, to which use it is often put.

In tourmaline, we have another stone which occurs in a large variety of colors, many of which are bright and attractive. The reds and greens are those which are mostly used in jewelry, and these transparent stones are comparatively cheap. Wider recogni­tion of this species would do much to make it more popular, for tourmaline has many qualities which make it most suitable for those who require color in jewelry of a moderate price.

The chemical composition of this stone is somewhat complex and it also varies slightly in different specimens, thus resulting in the different colors which one sees. But in general, it may be said that it is a silicate and borate of alumina, magnesia, and iron, with various traces of other elements. It is the only common rock-forming mineral that contains boron. Crystals are of the hexago­nal system, their general form being prismatic, often prisms of three sides, terminated with three or six faces. Vertical striations, often very obvious, are frequently seen on the crystal faces, and this may be sufficient to distinguish the rough stone in many in­stances.

Certain definitely colored tourmalines have been given their own names, but they are sometimes chemically grouped into three main divisions: the alkali, the iron, and the magnesium tourmalines. In the first group, we have the colorless, red, and green stones, all of which contain some sodium, lithium, or po­tassium. The second group is usually black, and the third group contains the yellowish-brown, the brownish-black, and some of the colorless stones.