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Smithsonian National Museum of Natural History


Pentahydroborite (NMNH G10624)
Photo by Ken Larsen. Click to zoom.
Catalog Number NMNH G10624-00
Locality Fuka Mine, Okayama, Japan
Weight 0.43 ct

Gift of Ms. Yuko Tanaka in 2009.

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 0.43ct gem is the first pentahydroborite for the National Gem Collection, and one of only two faceted gemstones known to exist.


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