gems, quartz, synthetic quartz, synthetic amethyst, hydrothermal, polysynthetic twinning

Different types of natural and synthetic citrine are characterized by a combination of absorption spectroscopy in the visible and ultraviolet range with microscopic investigations of structural properties such as twinning, growth structures, colour zoning, as well as orientated channel-like inclusions or orientated striations. Most natural citrines used in the trade are heat-treated natural amethysts; however, non-artificially-irradiated, non-heat-treated or heat-treated as well as irradiated and possibly heat-treated natural quartz is also found. Most synthetic citrines of the trade are different types of Fe-bearing as well as Fe- and Co-containing quartz crystals, which are grown hydrothermally from K₂CO₃-bearing solutions with or without additional oxidizers. Both methods reveal a number of typical features which indicate artificial irradiation and/or heat treatment. The colour of natural and synthetic citrine is caused by traces of iron and/or cobalt, as well as by radiation-induced colour centres. According to colour causes and treatment histories, natural and synthetic citrines are classified into different categories. Trace element analyses of synthetic citrine can indicate growth media and oxidizers used in the working solution.