OBSIDIAN
A common glass-like volcanic rock used extensively for tumbled gems, small carvings, ornamental objects, and jewelry; brittle and weak; black most com¬mon but also brown, reddish-brown, and streaked material, the so-called “flow” obsidians; Apache tears are black to dark brown clear nodules weathered from perlite rock; sheen obsidians contain numerous minute spangly inclusions, the best being a gold sheen from Mexico; also rainbow obsidian with colored in¬clusions, sometimes in pronounced bands with or without spangling; also spi-derweb, a brecciated type, and snowflake, a kind with grayish flower-like patches in a black groundmass. Available in large quantities and in large pieces to 10-25 lb. from California, Oregon, Nevada, Mexico.
OLIGOCLASE, See FELDSPAR
OLIVINE (Peridot)
Gem variety known as peridot; from dark brown to pale yellow-green, but only green hues used for gems; somewhat soft and brittle but customarily used in fine jewelry; finest crystals from St. John’s Island (Zebirget) in Red Sea but now scarce; good to fine large crystals found in Burma but color somewhat darker than St. John’s material; also, some contain swarms of very small inclu¬sions which may deaden the brilliance of faceted gems; in other instances the inclusions are aligned and furnish good chatoyant gems; Burma material is now unobtainable but small quantities of St. John’s peridot reach the market from time to time, while fair to good material from San Carlos, Arizona, con¬tinues to be in good supply although seldom capable of cutting flawless gems over about 5 ct.
ONYX, see CALCITE Or QUARTZ
OOLITE, see QUARTZ
OPAL
For a discussion of color and its bearing upon value in. precious opal varie¬ties such as black, white and Mexican types, see the following chapter; these value factors should be kept in mind when assessing rough. Additionally, rough sections should be turned over carefully under good light to see along which direction the most vivid color display occurs; in white opal, this direction is commonly upon the seam edges, but if gems were cut to put the best color “face up,” the stones would necessarily have to be small. Check all translucent mate¬rial against a strong light to detect opaque spots within; these may be areas of useless “potch” which will disfigure the finished gem. Check all Mexican, Ne-vadan, and Australian “jelly” opals against the light to see if cracks penetrate within otherwise clean areas; if so, these may enlarge with cutting due to relief of inherent stresses in the material; if possible, avoid buying such pieces. Ex¬amine all black opal seam material for thickness of seams; if such are too thin, the most delicate lapidary work may not be able to cut them into gems of suffi¬ciently broad surface area. Use glycerine for coating surfaces to be examined; hold stone at about arm’s length in a normally bright light. If the colors are quite apparent the finished gem will be bright and valuable. Most precious opal comes from Australia (whites & blacks); highly translucent to transparent color¬less, faintly bluish, yellow, brownish, and reds from Mexico, with or without play of color, some being facetable; also colorless, white, and gray to black from Nevada, with or without play of color; colorless transparent non-precious hyalite from Mexico; common opal is abundant but seldom used for jewelry; opalized wood is used for specimen slices, bookends, etc.
OPALITE, see OPAL
ORTHOCLASE, See FELDSPAR PECTOLITE
Fibrous white compact masses sometimes cut into cabochons displaying a weak chatoyancy; collectors’ gem only.