Mineralogical, geochemical, and gemological characteristics of silicic gemstone in AydA +/- ncA +/- k (Yozgat-Turkey)
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There are chalcedony and amethyst formations in two fields in AydA +/- ncA +/- k-Yozgat (West of AydA +/- ncA +/- k District, KeAYlik area and Southwestern of HacA +/- ilyas). The gemstones in this region have been formed within the gaps and cracks of the brecciated rocks of the fault zone in altered andesites. Chalcedony and amethyst are either present in the form of crack fillings from a few millimeters to 10 cm in width, or in the form of generally oval pore fillings reaching a few centimeters to 40 cm in diameter. The chalcedony is composed of 0.1-mm-centimeter-thick bands parallel to each other in varying tones ranging from light blue/white to dark blue to reddish brown, and in some points in the central part of this banded structure are large crystalline quartz and amethysts. Chalcedony occurs as kidney-banded and geodesic forms as cryptocrystalline and macrocrystalline. The altered volcanic rock with brecciated texture, which hosts chalcedony and amethyst occurrences in both regions, is the altered andesite. Petrographic examinations carried out from chalcedony and amethysts indicated that gemstones composed of fine-coarse grained quartz have microcrystalline texture in the rim zones and have macrocrystalline texture toward the core. XRD analysis of chalcedony and amethyst with different colors and crystal grain sizes revealed that gemstones in the region were composed of large and microcrystalline quartz. When amethyst, blue chalcedony, and reddish brown chalcedony were compared to white transparent quartz, there was a significant increase in the amounts of Fe2O3 and Al2O3, and metal elements (Mo, Cu, Pb, Zn, Ni, Co, Mn, Cr) in the case of amethyst and reddish brown chalcedony. The visible reserves of chalcedony and amethysts in the region, their dimensions and varying color range, and large-grained texture and physical properties show that the chalcedonies and amethyst formations can be used as gemstones. Use of these formations as gemstones also makes a significant contribution to the regional economy.