Topaz from mason county, Texas

Abstract

Gem-quality naturally blue topaz occurs in Mason County, Texas. Known as "Texas topaz," this gem is highly sought after in the United States because of its domestic provenance and rare natural blue color. To date, no studies have identified gemological or compositional characteristics that could distinguish Texas topaz from topaz sourced elsewhere. This study provides a historical account of topaz from Mason County and discusses its geologic origin, compositional characteristics, manufacture, and significance to the gem trade. A total of 83 alluvial topaz crystals from Mason County were analyzed using standard gemological testing; Fourier-Transform infrared, ultraviolet/visible/near-infrared, and Raman spectroscopic techniques; and laser ablation inductively coupled plasma mass spectrometry to gather gemological and geochemical data. Most Texas topaz is colorless, though some is light blue or brown. A small percentage of the blue stones displayed a saturated bodycolor. Color zoning was often visible and became more noticeable with luminescent excitation. Ultraviolet luminescence at 225 nm excitation revealed green to blue luminescent growth bands and rarely red luminescent growth bands. Topaz crystals from Mason County are not often free of inclusions. When present, multiphase melt and fluid inclusions contain carbon dioxide. Mineral inclusions include albite, anorthite, quartz, muscovite, pseudobrookite, and columbite-Tantalite. Machine learning algorithms were applied to fingerprint the trace element composition of the samples. Trace concentrations of phosphorus, scandium, titanium, iron, gallium, germanium, niobium, tin, tantalum, and tungsten best constrained the formation and provenance of Texas topaz. For gemologists, these trace elements can also aid in assessing the geographic provenance because most Texas topaz presents a distinct trace element composition. This work demonstrates the potential effectiveness of machine learning and statistical modeling in gemstone provenance determination. These can be powerful tools in conjunction with traditional gemological discrimination techniques.