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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.