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Physical Properties of Topaz
One of the best-known physical properties of topaz is its hardness. It has a hardness of 8 on the Mohs hardness scale, making it the hardest silicate mineral. Every student who takes a physical geology course learns about the exceptional hardness of topaz. Diamond, corundum, and chrysoberyl are the only commonly known minerals that are harder than topaz. Topaz occurs in a…
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Topaz – The November Birthstone
Topaz is the original modern birthstone for the month of November. Its status as a traditional birthstone contributes significantly to the gem’s popularity and is the reason behind many sales. Topaz jewelry is sold in almost every jewelry store in the United States. In 1952, Jewelers of America made a few modifications to the list of modern…
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What is Topaz?
Topaz is a rare silicate mineral with a chemical composition of Al2SiO4(F,OH)2. It is best known for being these four things: Topaz obtains much of its popularity from its beautiful colors and its status as a birthstone. Natural topaz colors include rare and valuable yellow, orange, pink, red, purple, and blue. The most affordable and frequently purchased are…
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Other Benitoite Deposits?
So far benitoite has been found at one place only. J. M. Couch, one of the original discoverers of the benitoite deposit, has located several prospects in formations resembling that at the benitoite mine. In one of these, three-fourths of a mile to the north on the east side of Santa Rita Peak, cavities lined…
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Benitoite Gemology
The mean refractive index of benitoite is greater than that of sapphire, and measures 1.757 to 1.804 (sapphire 1.759 to 1.767). The birefringence is high and the pleochroism very strong. The crystals are generally transparent with a pale to deep-blue and bluish-violet color. Color variations are common in the same crystal, and the change from…
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Chemical and Physical Properties of Benitoite
The chemical and physical properties of benitoite and its associated minerals have been described by Louderback and Blasdale, and the following notes are taken from their description. The chemical analyses show it to be an acid barium titano-silicate corresponding to the formula BaTiSi3O9 . Benitoite is insoluble in ordinary acids, but is attacked by hydrofluoric acid…
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Obtaining Benitoite Specimens
The benitoite is obtained by breaking open masses of vein rock and carefully chiseling or working the crystals out of the inclosing natrolite. Many gems are injured or ruined by this method. The removal of the natrolite by acid has been tried with partial success. Large slabs of rock 2 to 3 or more feet across are obtained coated…
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Mineralogy of the Benitoite Zone
The benitoite occurs with neptunite in crusts, seams, and thicker deposits of white natrolite on the walls of geode-like cavities and fissures in the hornblende schist. These deposits occur in both irregularly shaped masses and in seams with more definite directions. They inclose fragments of hornblende schist which has been heavily impregnated with natrolite. In some…
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Development of the Benitoite Mine
Development work at the benitoite mine at the time of the writer’s visit consisted of a large and a small open cut, a prospect drift or tunnel with a crosscut tunnel, and an incline shaft. The large open cut or “glory hole” was 20 to 45 feet wide, 85 feet long, and from a few…
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Geology of the Benitoite Deposit
The benitoite deposit occurs in a large area of serpentine which extends many miles northward past the New Idria quicksilver mine and a few miles southward, and forms the summit of an anticlinal ridge pitching down to Coalinga. This serpentine is of the usual type of the Coast Ranges and presents different phases from hard dark-green and…