WorldWide Drilling Resource

37 WorldWide Drilling Resource ® NOVEMBER 2016 Topaz is the November Birthstone by Harry W. Short, Engineering Geologist Treasured for its color, topaz has been used for centuries as jewelry. Larger gem varieties of topaz usually occur in granite pegmatites, but fine crystals have been observed in volcanic rocks and vein de- posits. The most desirable color for topaz gemstones is totally colorless, to gold, orange, blue, and pink. Heating topaz or exposing it to radiation can make dramatic changes in its color. Sometimes, blue topaz is sold as a substitute for aquamarine, and a yellow or brownish variety of quartz is also sold as a substitute for topaz. Yellow-colored citrine is sometimes also sold as topaz. A primary characteristic of topaz is its rarity of occurrence in a massive form. Usually, beautifully developed, sparkling, pristine crystals are found ranging in size from a pinpoint to rare crystals weighing hundreds of pounds. Crystals appear as short and stubby prisms which are diamond- or square-shaped in cross section. Perfect nat- ural topaz crystals have highly lustrous faces and appear as though they've been mechanically polished. To identify gemstones, it is sometimes necessary to look closely at the mineral's properties, such as hard- ness, crystal shape, color, specific gravity, index of refraction, and cleavage. In some instances, it's necessary to use chemical tests or even x-rays. The hard- ness of topaz, 8 on the Mohs Hardness Scale, is greater than most of the common minerals and its specific gravity is 3.4 to 3.6. Chemically, topaz is composed of aluminum, fluorine, and silica. There is little jewelry made before the 18th century which has survived, except perhaps those of ancient Egypt found in the pyramids. Early gem col- lectors mined in alluvial and gravely areas, such as streambeds with a stick or crude implement, where digging was easy. It has been reported that in the 17th century, a large colorless stone, called the Braganza diamond, weighing 1640 carats (one carat weighs 1/5 or 0.2 of a gram) in the Portuguese crown was actually a colorless topaz. This may have been the first gemstone hoax perpetrated in the world. The world's principal topaz localities include Russia, Australia, Tasmania, Brazil, Mexico, Pakistan, Japan, and the United States. You may be able to find more information about gem col- lecting areas from your local lapidary or rock/gem club. Harry In memory of Harry Short. His articles will continue posthumously. Contact: michele@ worldwidedrillingresource.com January Issue Deadlines! Space Reservation: November 25 th Display & Classified Ad Copy: December 1 st