Diaspore

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Diaspore, 6.3 cm, from Corundum Hill, Unionville, Chester County, Pennsylvania. Diaspore, an alteration product of corundum, was first discovered at Unionville around 1865 by William W. Jefferis. Jefferis reported in 1892 that a coating of well-crystallized diaspore crystals to 5 cm was found on a solid mass of corundum. The crystal shown here is from the collection of Clarence S. Bement (1843–1923), now in the American Museum of Natural History. Whether it is from the Jefferis pocket is not known, as there have been other accounts of fissures lined with diaspore crystals.