Neptunite is a silicate mineral with the formula K 2+)2Ti 24. With increasing manganese it forms a series with mangan-neptunite. Watatsumiite is the variety with vanadium replacing the titanium in the formula.
It was first described in 1893 for an occurrence in the Narssârssuk pegmatite veins within serpentinite in San Benito County, California, US. It also occurs in Mont Saint-Hilaire and in the Kola Peninsula
The mineral is named for Neptune , Roman god of the sea because of its association with aegirine , the Scandinavian sea-god.
The Gemological Institute of America (GIA) identified an 11.78-carat faceted specimen as neptunite based on Raman spectroscopy