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A very rare Stibiotantalite gemstone from Kunar, Pakistan.
Stibiotantalite is a complex mineral consisting of Antimony, tantalum/niobium and oxygen, chemical formula Sb(Ta,Nb)O4.
Its metal-rich composition is responsible for its unusual gemmological properties, with an "over the limit" RI of 2.37-2.45 and a high density (specific gravity) of 7.34-7.49. It also has a high dispersion and a hardness of 5.5 on Moh's scale.
This gem has excellent transparency and is only moderately included for the species.
It benefits from an electric and saturated yellow colour, with faceted examples of this quality being very scarce.
weight: 1.37 carats
shape: Cushion shape cut
dimensions: 5.40 x 4.98 x 3.10 mm