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In the Isle of Man, white quartz pebbles were put into Bronze Age graves and this continued up to the 17th century. If rubbed together, even under water, they give out a luminous glow, which may account for their sepulchral use.
(photo http://bit.ly/1fyhw8U; text Manx Calendar & Customs (1942) by Cyril Ingram Paton)
