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These two materials are not strictly minerals, but they may be seen on rare occasions in mounted jewelry of small value. The two species may be best described as glasses since they are not crystalline.

Obsidian is a lava which has cooled so rapidly in its formation that crystals have been unable to take shape. In composition and in many of its physical properties, obsidian greatly resembles manufactured glass. Its composition is variable and complicated, but it is largely made up of silica and alumina. Small quantities of potash, soda, and iron oxide are also generally present.

This material is easily broken, and since it has no crystalline structure, its fracture is uneven and conchoidal. Luster is vitre­ous, hardness is 5, specific gravity 2.3 to 2.6, and the refractive index, which is single, is approximately 1.5. Generally transparent, thick pieces do not transmit much light since the material is often smoky and dark, a dirty black being the normal color, although brown, gray, red, and green are also found. Some specimens which occur in North America, e.g. in Arizona, are striped like ma­hogany, while others which contain inclusions show a shimmering appearance. This last type is fairly rare and therefore is considered of some slight value.

Although comparatively soft and easily broken, obsidian pre­sents difficulties to the lapidary which are not compensated by the commercial value of the cut stone. It is therefore seldom seen in jewelry, jet and other black stones having displaced its use in articles of mourning. In ancient times, it was commonly used as a weapon of war, and knives, spear and arrow heads with a very sharp edge were laboriously ground from the rough stone. We read of such weapons being used extensively by the Aztecs, and ancient   Mexican   cemeteries  are  prolific  with  relics  of  these

weapons, as well as vases and mirrors, all of which were made from locally found obsidian.

The Spaniards under Cortes had cause to dread these weapons, made from the native itzli, which the Aztecs used against them. The hard, vitreous stone was brought to a sharp edge, and it made a formidable weapon when set in a javelin or in a spear. Various arrow points and needles fashioned and made from obsidian found in Oregon, U.S.A., have been discovered in the original mines (Glass Butte, Lake County). The mineral was evidently greatly esteemed by the native Indians here, and the stone is still worked. Various legends and superstitions surround this attractive glass, and it was used in the ornamental dress of the White Deerskin Indian tribe when they celebrated their biennial dance.

Pliny also mentions that this material was used for plates and dishes, as well as for mirrors, but it must be noted that the Romans used artificial glass for the same purposes, often calling it obsidian.

Obsidian is found in nearly every volcanic region, but the islands of Lipari (off the Italian coast) are the chief European source of worked material. Quantities exist in Hungary, Mexico, Nevada (U.S.A.), Siberia, and in other districts. A meteoric glass from Czechoslovakia sells under the name of "bottle stone" or "water chrysolite," and is very similar. Apart from obsidian and moldavite, there are other natural glasses. These may be grouped as basalt glasses and tektites, but they are not used in jewelry.

Moldavite is a dark green, transparent material possessing the physical characters of artificial glass except that it contains more silica and no potassium. Of a dirty bottle green color, it is some­times confused with chrysolite, but being singly refractive and having a hardness of only 5 1/2, with a specific gravity of 2.36, it should be easily distinguished from that stone by simple tests. Refractive index is approximately 1.50. Actually, its composition is 75 to 80 per cent silica, with 10 per cent alumina and traces of other elements.

Its origin is somewhat doubtful, although now it is generally accepted that it is a meteoric substance. Examination under a microscope often shows a number of small bubbles or pores, and also peculiar surface markings. Since it is amorphous, the material is single refractive and not dichroic. It is found chiefly in Czecho­slovakia (Bohemia), not far from the surface of the ground, in the form of roundish or oval shaped lumps, but the gem gravels pf Ceylon also yield specimens,

Stones are very rarely cut, and the usual color, a dark bottle green, is brought to light only in polishing. This mineral, often marred by bubbles and striae, is sometimes called water chrysolite, pseudo-chrysolite, and bouteillenstein, and it is not infrequently sold, incorrectly, as real chrysolite. Its general appearance some­times leads one to confuse it with obsidian, but it has several different physical characters from this other natural glass. Its name is derived from the original source of its production, the basin of the river Moldava in Czechslovakia.

Another silica glass similar to these two already mentioned has been found on the surface of the Libyan desert, about 500 miles southwest of Cairo. Wind-worn lumps weighing up to 10 lbs. have been found in hollows between the sand dunes, some specimens having been brought back by an expedition of the Egyptian Desert Survey.

This silica glass is of a pale, greenish-yellow color, sometimes being quite transparent. Under the microscope, it is seen to con­tain a huge number of minute bubbles and impurities. Its refrac­tive index is 1.44, the specific gravity varies from about 2.20 to 2.22, and the hardness is 6. Chemically, it contains about 97.6 per cent of silica. If cut, this stone somewhat resembles olivine, but it lacks brilliancy. A cut stone at the British Museum (Na­tural History Section) in South Kensington, London, weighs 52.86 carats, and another is in the possession of a late king of Egypt.

This stone was once used in mourning jewelry, like jet, but now it is again seen in mounted forms, but mainly in districts where the rough stone is mined, and where it is sold as a "local" stone. In color, hematite is black with an iron, metallic luster, although thin pieces appear by transmitted light to be transparent and tinged a blood red color. This property gives us the name for the mineral, the corresponding Greek word (haema—blood), meaning bloodstone.

It is an iron sesquioxide, or ferrous oxide, containing 70 per cent of iron and 30 per cent of oxygen, when pure, but clayey and sandy impurities are sometimes present. Hematite crystallizes in the hexagonal system, although when properly so called, it is granular, reniform, or amorphous, the crystallized variety being specular iron. The common forms of the crystals are rhombohe-drons. Cleavage is parallel to the faces but generally indistinct, and hardness is 5  1/2 to 6 1/2. Specific gravity varies from 4.5 to 5.3, and the refractive indices are 2.94-3.22.

Although opaque except in very thin plates, a highly metallic luster is exhibited. Beneath the surface of the reniform variety, or kidney ore, a radiating columnar structure is often visible. A streak of a dull red color when rubbed on a piece of unglazed porcelain or ground glass is a test for this stone. Magnetite would leave a black streak, stibnite a gray, and pyrites a greenish black mark. It is harder than jet, yet softer than onyx, and it is really not suited for use in jewelry. But it is cut, sometimes in intaglio form, and the mounted jewelry can be seen more generally in Spain, France, Mexico, and some southern American countries.

A German term meaning "sea-foam," this material somewhat resembles in appearance the froth sometimes seen on sea water. It is amorphous, very light and soft, and a dirty white, grayish, or light yellowish in color. The French also know this stone by the name ecume de mer—sea-foam.

Meerschaum has no outstanding qualities of great merit. It is opaque with an earthy texture, porous, with hardness of only 2 to 2 1/2, and specific gravity of about 2. Chemically, it is a hydrated silicate of magnesium. A doubtful advantage it possesses is its property of adhering to the tongue, this being due to its porous nature and its peculiar texture.

Meerschaum is found in Turkey, Greece, Spain and Morocco, but most of the worked material comes from Asia Minor. Here, it occurs in alluvial deposits as nodular masses of irregular shapes and varying sizes. It is systematically worked by digging pits and galleries; the earthy matrix is collected and scraped, and the lumps are then dried and scraped again, after which they are boiled in a wax and polished. When first unearthed, meerschaum has a greasy feel and lathers like soap. On this account, it was for long used by the Tartars for washing their linen. If heated, meerschaum gives off water and a fetid odor, subsequenty hardening and becoming perfectly white in color.

The meerschaum industry at Eskischir in Turkey has in recent years experienced something of a revival. Some 621 metric tons were produced in 1935, but output varies considerably with eco­nomic factors. For instance, there was always a good demand in Hungary for large meerschaum pipes, but during the last few years, unsettled conditions has temporarily put an end to that business.

Like alabaster, marble is an ornamental stone used chiefly in architectural and large decorative work. Some variety of marble is found in almost every country. Its uses are well known, but its properties are not so generally recognized.

Marble is really a metamorphosed limestone, a carbonate of lime, with hardness of 3, specific gravity 2.71, and refractive in­dices 1.48-1.65. When pure, it is white in color, but it is more often streaked on account of included impurities, such as iron oxide. Nearly every color is quarried, and each variety is known by name according to the formation in which it occurs, its place of origin, or its peculiarity of color. Its structure is granular, and the interlocking grains of calcite, of which it is formed, may be easily seen if examined with a microscope.

Although marble cannot be regarded as a gem stone, its beauty, comparative durability, and its low cost have enabled it to be used as a material by which the sculptor and the architect have ex­pressed their ideas of beauty in consummate form. Both in Italy and in Greece, marble has been used as a medium by sculptors throughout many centuries.

The buildings that still stand in Athens, such as the Parthenon, the Erecthelium and Propylaea, are composed of marble, as are also such outstanding works as the Hermes of Praxiteles and the Aphrodite of Melos. Many more modern buildings of outstanding beauty have been built of this material; for instance, nothing but marble was used in the build­ing of Milan cathedral. There are a great many varieties of mar­bles; we shall mention a few as being of interest since they may be seen, either inside or outside many of the finest buildings existent.

Among the opaque minerals which we are about to describe in this chapter, alabaster is one of those which is never mounted in jewelry, its use being generally confined to building and decora­tive work. But like all the species which follow, it has some dis­tinguishing features which raise it above ordinary rock.

A hydrous calcium sulphate, alabaster is crypto-crystalline in structure and generally white in color, although it is often streaked with reddish impurities. Browns and yellows sometimes occur; all are opaque, with a greasy luster. Nevertheless, some degree of polish may be obtained.

Alabaster is, however, very soft compared with most gem stones, reaching only 1 1/2 to 2 on the scale of hardness, and it may be scratched with the finger nail. Specific gravity is about 2.3 and it is slightly soluble in water. Suitable material is cut into a variety of shapes, such as lamp shades, without much difficulty.

Material found near Volterra in Italy is of a rich brown color, sometimes passing to a lemon yellow, and this is much esteemed. Florence has a considerable trade in this stone, but it is also found in Switzerland, the Tyrol, and the U.S.A. Very ancient quarries exist in Algeria, from whence it is thought that the ma­terial mentioned in the Bible originated. On the other hand, it may have been quarried in the neighborhood of Thebes (Egypt), which still produces useful stones.

Alabaster was used by the ancients for making small perfume bottles. The Romans used to make vases from this material to hold the tears which widows shed on the death of their husbands; hence they were called lachrymatory vases. Also cinerary urns were made, in which ashes of the dead were preserved.

The term alabaster may be derived from the Arabic al bakstra-ton, meaning "the whitish stone."

There are three other substances which somewhat resemble alabaster in chemical composition—selenite, satin spar, and gyp­sum. All belong to the monoclinic system of crystallography; their hardness is about 1 1/2 to 2, and specific gravities are about 2.30. Selenite (derived from a Greek word meaning moon, in allusion to its pearly luster), is a crystalline form of sulphate of lime. It may be easily split with a knife into thin, transparent layers or plates. Such plates have been used as window panes, especially in Bolivia.

Satin spar is a fibrous variety of massive gypsum which is found in Derbyshire, Nottinghamshire, and in parts of Glouces­tershire. It is softer than cat's-eye, which it somewhat resembles. A material often called satin spar is a white, fibrous variety of limestone with a satin-like luster, moderately hard, and some­times carved and worked in the form of boxes and other small ornaments. It is really a carbonate of lime, a form of calcite, and it is found in veins or crevices in rocks, the fibers stretching across the crevices.

The massive form of sulphate of lime is called gypsum, and this, when heated to expel the water and then ground to a powder, is the Plaster of Paris so well known in commerce. It is a white powder, granular and compact, and almost insoluble in water. When mixed with a small quantity of water so as to form a thin paste, it gradually thickens and soon solidifies into a hard mass of hydrated sulphate, the water being absorbed. On account of this property, it is of great use in taking casts and molds of objects; it is also used to some extent as a plaster, although the climate of some countries renders it unsuitable for that purpose. Gypsum is widely distributed over many countries.