A number of natural glasses have been used from time to time for gem purposes, the most common of which is obsidian (ahb-SID-ee-an). The Romans called it "obsinnus", since it had the same appearance as a stone discovered in Ethiopia by a citizen of Rome names Obsius. In all of the early editions of Pliny's "Natural History" the name appeared as "obsidianus", and eventually it was shortened to the present spelling.
Obsidian varies from transparent to opaque and ranges in color from black through brown, red, yellow, and gray to slightly greenish or bluish. In its most common form, however, it is opaque to semitransparent black to dark brownish red. Rock collectors distinguish a number of varieties; e.g., banded obsidian (material with bands arranged somewhat similarly to agate, except that they are usually sinous rather than curved), onyx obsidian (straight, parallel bands), and flowering obsidian (white patches of crystallized silica in the black groundmass). Some varieties exhibit a sheen in certain directions, caused by minute, highly reflective inclusions. Small, rounded, irregularly shaped pieces, usually translucent and light to dark gray in color, are called "Apache tears".
Obsidian is widely distributed throughout the world. Notable sources include Mexico, New Zealand, Iceland, Hungary, Greece, California, Oregon, Nevada, Arizona, Wyoming (Yellowstone National Park), and other western states. It occurs in sizes from small nodules to sheets measured in hundreds of feet. It is formed by the rapid cooling of magmas under low pressure near the surface, usually when molten lava encounters a lake or other body of water and cools rapidly.
The usual composition of this natural glass is essentially that of granite; i.e., 60% to 75% silica, with some alumina, alkalis and iron. It is amorphous, has a vitreous luster, a hardness of 5 to 5 1/2, fairly good toughness and, of course, no cleavage. The fracture is conchoidal and the S.G. varies from 2.34 to 2.46. 1.50 is the usual R.I., but it may vary from 1.48 to 1.52. Obsidian is attacked by hydrofluoric acid and is fusible. Since it is isotropic, birefringence and pleochroism are absent.
Moldavite (MOLE-dav-ite) is the most important of a group of natural glasses known as tektites (TEK-tites), which are thought to be of meteoric origin. The name tektite is from the Greek meaning "melted". Moldavite, which often resembles the yellowish-green color of peridot, takes its name from the Moldcu River in Czechoslovakia, where it was first found in 1787. It may also be grayish green or dark to medium green. Other glasses of the apparently same meteoric origin have been discovered, but only moldavite has been used as a gemstone. These stones have been found in a number of other places throughout the world, including the Libyan Desert, Texas, Western Australia and the East Indies. They usually occur in flat or rounded shapes smaller than a man's fist, and are always characterized by a wrinkled and scarred exterior. The properties and composition of moldavite are approximately the same as those of obsidian.
Difficulty may be encountered in separating the natural glasses
from their artificial counterparts. For example, although most
manmade material has different properties than the natural, it is possible to make glass that has the same R.I. and S.G. Sometimes the inclusions of the natural may lead one to suspect its origin, but this is not a dependable means of separation, since moldavite usually contains gas bubbles and flow lines. However, moldavite may be distinguished from other stones that may resemble it, such as tourmaline and peridot, by the dichroism and birefringence of these doubly refractive stones and by the bubbles in moldavite. In addition, such isotropic stones as demantoid may be separated on the basis of their high luster and S.G. Obsidian is considerably lower in R.I. than jet and less dense than other black minerals it may resemble; i.e., the black variety of tourmaline, schorl.
Patterned and semi translucent material is cut in cabochon, and transparent stones are sometimes faceted. The natural glasses are brittle and sensitive to the heat generated during the fashioning operation; for this reason, wet sanding is recommended. Cerium or tin oxide on a felt lap is recommended for cabochons, and either tin oxide or Linde A on a tin lap or cerium oxide on a lucite lap produce good results on faceted stones. Recommended angles are 42° for the crown and 43° for the pavilion.
The natural glasses have very little value, seldom being priced above the artificial material. The exception to this is the tektite that has been publicized and sold as "ngni mnni" or "fire pearl". Masquerading under these fanciful and misleading names, the stone has been offered at ridiculously high prices.