Benitoite is a rare, strongly dichroic, blue mineral used as a gemstone . It is found in significant quantities only near the head waters of the San Benito River, San Benito County, California. In spite of much effort to study benitoite, the origin of color in benitoite has not been definitively established. Because traces of Fe are found in benitoite, ideas proposed include the Fe2+ - Ti4+ or the Fe2+ - Fe3+ intervalance charge transfer. While most benitoite is colorless when viewed down the c-axis, there is a very small number of exceedingly rare stones (now cut into gemstones) which are pink in this direction. All of the pink stones came from the same small pocket in the mine. Heat and irradiation of blue stones does not change their color. Irradiation of colorless stones does not change their color, but heat treatment can make them turn orange.
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