Mineralogical, Geochemical and Gemological Characteristics of Chalcedony from the Ergani Region (Diyarbakır, Türkiye)

In this study, the mineralogical, petrographic, geochemical, and gemological characteristics of siliceous gemstones exposed in the Ergani (Diyarbakır) region were investigated. The siliceous gemstones occur within Eocene sedimentary units, filling karstic cavities and structural discontinuities over an area of several square kilometers. Chalcedony bodies reaching volumes of several cubic meters occur in green, blue, red, and white tones and locally contain Fe-oxide minerals (goethite–hematite), clay minerals, and carbonates as pore fillings and/or micro-inclusions. Thin-section analyses indicate that the samples are predominantly composed of microcrystalline quartz and chalcedony, exhibiting a well-defined cavity-filling sequence. This sequence is characterized by the precipitation of fibrous chalcedony along cavity walls, followed by microcrystalline quartz and, locally, coarser crystalline quartz toward the center. Chalcedony commonly displays complex intergrown and interlocking (grift) textures rather than simple banding, reflecting dynamic and multi-stage mineral precipitation processes. XRD analyses confirm quartz as the dominant mineral phase, while dolomite, hematite, and clay minerals are identified as accessory components, particularly in the red chalcedony samples. Geochemical data show high SiO2 contents (77–97%), while enrichments in trace elements such as Fe, Cr, Mn, and Ni indicate significant hydrothermal fluid–rock interaction, likely involving mafic–ultramafic sources. Colouration is primarily controlled by Fe-oxide inclusions and transition-metal enrichment, whereas relatively pure silica precipitation accounts for the formation of white chalcedony. Overall, the results demonstrate that the siliceous gemstones of the Ergani region formed through a multi-stage hydrothermal silicification process controlled by structurally mediated fluid circulation and evolving physicochemical conditions. These findings are consistent with a conceptual genetic model involving repeated fluid influx, cavity-filling mineralization, and coeval precipitation of silica and accessory phases. Considering their mineralogical purity, complex textures, colour diversity, and workability, these occurrences represent significant potential from both scientific and gemological perspectives.