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Pentahydroborite is a very rare borate mineral. It is a soft stone, only 2.5 on the Moh's scale of hardness. It typically has a vitreous luster and is transparent and colorless. It has been found in Japan, Russia, and Serbia. Pentahydroborite is extremely difficult to facet, and the rough is usually not suitable for cutting. This is the first pentahydroborite for the National Gem Collection, and one of only two faceted gemstones known to exist.

Image Number: Ken Larsen
Catalog Number: G10624
Weight: 0.43 carats
Gift of Yuko Tanaka , 2009
Locality: Fuka Mine, Fuka, Bicchu, Kawakami, Okayama , Japan


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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits