Quartz with Inclusions
Inclusions of many minerals have been observed in coarsely crystalline quartz, but only a few are sufficiently abundant to color the mineral or large enough to be seen with the unaided eye.
Sagenitic quartz, or simply sagenite, is a term applied generally to any quartz variety that contains eye-visible acicular crystal inclusions. Rutilated quartz, sometimes called "Venus hair stone," is rock crystal containing reddish brown to golden needles of rutile that are usually randomly oriented (Fig. 14.44). Occasionally they are found running fairly parallel to each other; when cut en cabochon, such stones exhibit a weak chatoyancy. Fibers of green ac-tinolite (actinolated quartz) and long slender crystals of black tourmaline (tourmalinated quartz) are also found penetrating colorless quartz.
Quartz enclosing closely packed, parallel asbestos fibers has a chatoyancy when cut en cabochon. This cat's-eye quartz somewhat resembles chrysoberyl cat's-eye. The best examples come from Sri Lanka and India. More recently some brownish green material has come from Brazil that exhibits one strong chatoyant band intersected by a much weaker second band. Some of this material is better designated as a form of asteriated quartz.
The tiger's-eye variety is quartz that has replaced closely packed, parallel crocidolite asbestos fibers. It exhibits a broad chatoyancy even on flat surfaces that is accentuated when cut en cabochon. The color ranges from brownish yellow to brown to reddish brown. Material that retains the original blue color of the crocidolite is known as hawk's-eye or falcon's-eye quartz, while material with both the blue and brownish colors is sometimes called zebra tiger's-eye. Tiger's-eye may be heat treated to produce a brownish red color through the dehydration of iron oxide impurities. South Africa is the major commercial source. Tiger's-eye is commonly used in beads, cabochons, and carvings.
Aventurine is the name given to quartzite that contains small platy inclusions of mica that impart their color to the otherwise colorless mineral. A green color results from flakes of fuchsite, a chromium-bearing muscovite mica, while reddish brown is due to spangles of mica of that color and resembles the aventurine varieties of feldspar. A brown glass, known as "goldstone," containing crystals of copper is manufactured and could be confused with aventurine quartz or feldspar. It is softer than either of these, and the reflections are more flashy than in the natural material. Green aventurine quartz comes principally from India and is fashioned into beads, cabochons, and ornamental objects; it is sometimes referred to by the misnomer "Indian jade."
from "Gemology," Second Edition by Cornelius S Hurlbut, Jr. and Robert C. Kammerling. Copyright © 1991.
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