The term "topaz" is often loosely applied, both by the general public and by dealers in gem stones, to many stones that are not topaz. One generally visualizes topaz as being a yellow stone, and this is certainly its characteristic color. Yet the true topaz, or precious topaz as it is sometimes called, is found in a variety of colors. It is not a common stone, and it is not widely used in jewelry. What most people know as topaz is really the commoner stone, citrine or yellow quartz, and it is unfortunate that the term is not strictly kept to the more beautiful and rarer species.

Chemically, topaz is a fluo-silicate of aluminum, and it occurs in natural crystals, prismatic in habit with pyramid terminations. These crystals fall into the rhombic system of crystallography, al­though there is some doubt as to the real nature of the crystal structure of this stone. Some think that it belongs to the pyrami­dal polar class of symmetry; others think that its true habit is holohedral, that is, with doubly terminated crystals fully devel­oped at both ends. Crystals usually have sharp edges and well defined faces, with a high natural polish on their surfaces. They are found in sizes weighing over 100 lbs. down to quite micro­scopic pieces. Twin crystals showing intergrowth between rock crystal and topaz are common.

Of the many gem stones known, topaz is one of the very few which contains the element fluorine; water is also present in a minute quantity. Many specimens have been found to contain cavities of varying sizes, which are filled either with gaseous liq­uids or small crystals of foreign matter, such as ilmenite, hematite, and rutile.

Topaz occurs in a wide range of colors, but the commonest are pure white and those with pale yellowish shades. Very beautiful limpid yellow and sherry shades are characteristic, but pale blue —a lighter and harder, steely looking blue than the aquamarine and also pale greens are also seen on the market.

As a gem stone, chrysoberyl is comparatively uncommon. Rough material is scarce, and good specimens reach a fairly high price. Yet it is not a stone that attracts everyone, despite its many qualities which make it quite suitable to be used in jewelry. There are two varieties of chrysoberyl so used, the green and brownish-green transparent stones called alexandrites; and the cloudy, grayish-yellow, cabochon cut stones which show a chatoy ant effect, and which are named oriental cat's-eyes, or cymophanes. Both varieties have striking characteristics which easily distin­guish them from all other gem stones.

The dark green alexandrites are the most sought after variety, and good cut specimens very seldom exceed five carats; in fact, the few stones one sees in the market are almost always small and under one carat. Clear, unmarked stones of a good color realize very high prices from the connoisseur, for they exhibit the unique property of changing color by artificial light. In this light, the leaf green changes to a soft raspberry red, this being due to the greater absorption of the red part of the spectrum by artificial light. Alexandrite is strongly dichroic, and in ordinary light the violet and yellow rays are mostly absorbed, the transmitted light being mainly composed of the red and green portions of the spectrum. In artificial light, which is relatively stronger in the red part of the spectrum, a specimen will appear to have a pale rasp­berry red tint; in daylight, it will be predominantly green.

The greenish-yellow and pale yellow transparent stones are often called chrysolites, although this term is properly applied to the pale green peridot. The two stones, which are somewhat superficially alike, may be easily distinguished by the greater hardness and the higher specific gravity and refractive indices of the chrysoberyl.

Aquamarine is the name applied to the bluish beryls. In other words, it is exactly the same stone as an emerald, except for color,

and it has all the same physical properties such as specific gravity, hardness, and refractive indices as its rarer companion. The fact that aquamarine is found more abundantly and often in large and flawless crystals makes it a much less costly stone than em­erald. Pale stones are comparatively cheap, especially those which approach being colorless, but good blue stones, which are very beautiful, are fairly rare and obtain high prices.

Besides the dark green beryl, which we know as emerald, and the blue beryl, which is the aquamarine, we have other colors in this mineral species (a silicate of beryllium and aluminum) and these are called beryls only, according to their colors. The min­eralogist regards the whole class simply as beryl, but as we are concerned with jewel stones, we must make some distinction when describing what is more properly gem material and not ordinary rock. We therefore have yellow beryls, sometimes known as golden beryl and heliodor; the rose pink, called pink beryl or morganite (after J. Pierpont Morgan of New York who presented a large collection now on display at The American Museum of Natural History in New York); white beryl, also called goshenite; green beryl, and other transparent varieties whose colors may be intermediate or indefinite, and which are consequently known simply as beryls. There are dull, cloudy stones, some even opaque, and these are not cut for use in jewelry.

But all these crystallize in hexagonal columnar crystals. Cleav­age is indistinct, and dichroism is weak, except in the deeper blu­ish and greenish stones, when twin colors are distinctly seen. Hardness is almost universally 7 3/4, specific gravity about 2.74, and refractive indices 1.57-1.58. Aquamarine is the lightest of the beryls, many stones giving a reading as low as 2.69; the pink beryl, on the other hand, sometimes reaches 2.80. The various colors are due to traces of impurities.

Pink beryl is a beautiful, true rose pink, and it is the largest pink transparent stone found. It is very pure and clear, with less of a magenta tint than tourmaline and without the lilac color seen in kunzite. Aquamarine, which does not change in artificial light, is not cloudy or heavy looking like most blue zircons.

This term really includes a large group of minerals, isomorphic in character, but only a few of them are used in jewelry. Mag­netite, the strongly magnetic iron ore, chromite, a chrome ore, and gahnite, a zince aluminate, are among the several minerals which are classed within the spinel group, but the characteristic gem spinel, which excludes these minerals, is of a brilliant color and clearly transparent, especially in artificial light. Although not com­mon, it is found in a variety of colors, and it is often confused with other stones when cut and polished.

All the varieties crystallize in the same system (cubic) and the chemical constituents are analogous although the actual composi­tion differs. The natural crystals occur commonly as octahedra, which are often twinned. Chemically, spinel is a double oxide of magnesium and aluminum.

The particular group of spinels used in jewelry furnish stones which are often very attractive in color. They are sometimes called precious spinels, and almost every shade is known to exist. Their wide range of colors may be compared to the fancy colored sap­phires, colorless specimens being somewhat rare. Many inter­mediate shades are seen, and all are due to different metallic oxides. The color may be sometimes improved by careful heating. The coloring agent in blue spinels, for instance, is iron. In these spinels, sometimes called gahno-spinels, zinc sometimes replaces the magnesium, particularly in specimens which come from Cey­lon. For that reason, the refractive index and the specific gravity will be found to be slightly higher than normal.

The oldest known opal mines are now in Czechoslovakia, al­though this area was formerly Hungary, and hence we have the old term Hungarian opals, which is now rarely used. These mines were the source of supply for the Romans, and even in those days

opals were highly appreciated. Pliny describes them in glowing terms, and long before his day, the Greek writer Onomacritus re­marked that "the delicate colors and tenderness of the opal remind one of the loving and beautiful child."

Throughout the Middle Ages, the^old Hungarian mines were worked intermittently, sometimes by private enterprise, some­times under government control, but no large quantity of stones was ever produced. In the early years of this century, the industry employed not more than sixty men as the deposits were almost exhausted. In fact, during the last fifty years, the output has been negligible and commercially of no importance. No stones, except some of a small size, have been mined here for the last thirty years.

Almost all opals, with the exception of fire and water opals, now originate from Australia. Although certain fields have already been exhausted, others are constantly being discovered in that conti­nent. There are doubtless many unknown and undeveloped de­posits, and as the mining is left to the hardy and adventurous individual, wars and sudden demands for other minerals adversely affect the supply of opals. The Australian deposits are mostly lo­cated in very inaccessible areas, and supplies are irregular.

The varieties of opal found in the rough are divided into four main classes, boulder, sandstone, seam, and black opal. Boulder stones occur as thin veins, brilliantly colored, in gray and brown ironstone, and they are found at varying depths. When freed from the matrix, the precious stone is said to ring like a metal if struck. Material found in soft ground is generally inferior in quality to that found in the harder rocks. This type of opal was first mined in West Queensland in the year 1875, but now very little is found.

This gem stone is definitely different from all others, even super­ficially, and it cannot be confused with any other stone, either in the rough or cut state. Moreover, its imitations are almost always obvious, and owing to its manner of formation and its composi­tion, synthetic manufacture is practically impossible. The char­acteristic play of colors with the remarkable flashes of red, green, and blue which the gem opal shows, and which, indeed, range throughout all the shades of the spectrum, make this stone unique. Fine specimens justly command high prices, and black opals rank in beauty and value with the precious gems.

Black opals are those having a blackish background, but show­ing a splendid mass of red or green flashes. They are rare. White opals, which are by far the more common, have a whitish back­ground, but unless they show a distinctly variable change of color, they are considered of little value as they are not very attractive. In fact, the milky white pieces are not often cut or used, except in very cheap forms of jewelry. Thus variety of color within the one stone is the chief attraction of opal, and it is the main fac­tor which determines its value. This is in contrast to most other gem stones, where uniformity of color is considered a necessary quality of perfection.

Although there are various kinds of opals, the chemical compo­sition is the same in every instance, with the exception of certain small impurities, chiefly metallic oxides, which may be sometimes present. Silica (oxide of silicon) is the chief constituent of opal, and a varying amount of water, from six to ten per cent, is always included. Metallic oxides do not cause the remarkable play of colors which all gem opals display. This is due to the peculiar internal structure of the material, which is not crystalline, but amorphous. It is the result of disturbed conditions while the opal is being formed within the earth from its natural constituents, silica and water.

The exact process by which Nature produces this gem is not, as yet, fully understood. It is believed by many authorities that it is formed from a silica gel, which sometimes shows a pseudo-crystalline structure, and that the play of color, known as opal­escence, is caused by a structure of curved thin films which pro­duce optical interference. If this is so, opal cannot be said to be entirely amorphous. On the other hand, some believe that the material contains very minute fissures caused by the jelly-like mass cooling very quickly, thus causing interference of light. In any event, the material does crack very easily, and often when slightly heated.