Dark Red Tantalite Crystal in Greenish Blue Sapphire

Axle Estrella and Joseph Hukins

The authors recently examined a 1.80 ct greenish blue sapphire and were surprised to observe a dark red internal protuberance jutting inward from the surface of the pavilion (see above). The prismatic inclusion was identified as manganotantalite ((Mn,Fe)(Ta₂O₆); a manganese-enriched iron-tantalum oxide) using Raman spectroscopy, consistent with its red bodycolor. This mineral displayed a blunt termination and etched pinacoidal surfaces. Laser ablation–inductively coupled plasma–mass spectrometry analysis showed that the host sapphire had iron levels of up to 1900 ppma. With a spectroscope, a strong broad absorption around 840–880 nm was observed. These observations are consistent with known occurrences of tantalite in sapphire hosts from basaltic-related origins (S. Promwongnan and C. Sutthirat, “Mineral inclusions in ruby and sapphire from the Bo Welu gem deposit in Chanthaburi, Thailand,” Fall 2019 G&G, pp. 354–369).

While tantalite itself is not uncommon in sapphires of this type, this inclusion was notable due to its interesting morphology and attractive dark red color, attributed to the presence of manganese, which contrasted with the greenish blue stone (O.I. Lee and E.T. Wherry, “Manganotantalite from Amelia, Virginia,” American Mineralogist, Vol. 4, No. 7, 1919, pp. 80–83).