Gemmological samples: rock crystal from two localities in the Rhodopes, Bulgaria

The paper comprises gemmological and other data on several specimens of faceted rock crystal gemstones from two localities in the Madan ore field, Smolyan Province, and Krumovgrad Municipality, Kardzhali Province, of the Rhodopes, Bulgaria.


Introduction
Quartz (SiO 2 ) is extraordinarily common mineral on Earth and occurs in a wide variety of rock types and geological environments. Fine specimens of its colourless crystalline variety, rock crystal, are known from many localities, especially in the USA, from Mount Ida, Montgomery County, Arkansas; Minas Gerais in Brazil; in the Alps of France, Switzerland, Italy, and Austria; Madagascar; Ural Mountains (Nether-Polar Urals) and elsewhere in Russia; Myanmar (Burma); Yamanashi Prefecture, Japan (Anthony et al., 2001(Anthony et al., -2005. While studying the rich mineralogical holdings of the National Museum of Natural History at the Bulgarian Academy of Sciences, Sofia (NMNHS), Karov & Dimitrov (2004: 9-10) list numerous localities and natural specimens of quartz and its varieties from different regions of Bulgaria, including many from the Rhodopes.
Rock crystal is frequently used in faceted gems, beads, carvings, and decorative objects. As a material for faceting, it has little intrinsic value, except in very large, flawless pieces. It can be of enormous size, as illustrated by the more than 32 cm in diameter, 48.5 kg perfect sphere of flawless Burmese material in the National Museum of Natural History, Smithsonian Institution, Washington, DC, the largest fine crystal ball in the world. There are faceted specimens of 7000 carats recorded (Arem, 1987: 158-159).
Faceted specimens of rock crystal or any crystalline quartz from Bulgaria, however, remain a certain rarity.
The crystalline quartz, especially rock crystal, is common in the mines of the Madan ore field and is often associated with calcite (CaCO 3 ), chalcopyrite (CuFeS 2 ), galena (PbS), pyrite (FeS 2 ), sphalerite (ZnS). Five generations of crystal growth have been identified there (Petrussenko, 1991: 444). It should be noted that the Madan ore field is a large lead-zinc mining district, known since ancient times, exploited in the Middle Ages and extensively mined during the last two centuries in more than 40 underground mines. Madan mines produce one of the finest quartz crystals in Bulgaria. In the East Rhodope Mountains, in the valley of Arda River and the nearby areas a cryp- tocrystalline variety chalcedony prevails, whereas crystalline samples are relatively rarely found in very small crystals in specific geodes (Tsintsov & Banushev, 2010) and are generally not suitable for faceting. The majority of the gemstones were cut on electronically modified variant of Xristal-tek "87" faceting machine by Poly-Metric Instruments, Inc., Clayton, WA, USA, using diamond coated flat laps of different grits (320, 600, 1200) by THK Diamond Tools, Hong Kong, and polished using a DARKSIDE™ lap (Gearloose Lapidary, LLC, North Easton, MA, USA), charged with zirconium dioxide BATTSTIK™ (Gearloose Lapidary, LLC) or cerium(IV) oxide powder mixed with distilled or low mineralisation water.
Some faceted samples from Krushev Dol Pb-Zn deposit contain tiny cavities or negative crystals with bubbles and other inclusions. They are generally very clean. These from Yaraklitepe are cleaner, but substantially smaller. In both cases, such a clarity can be considered natural, because crystal parts with inclusions are usually removed during the faceting process.
very much obliged to Associate Professor Svetoslav Petrussenko, Sofia for his valuable suggestions and corrections.