Neptunite

Big Photo

San Benito Co., California, USA
11.78 carats
© Rarestone.com

Neptunite is named after Neptune, Roman god of the sea, because it was found with aegirine, which was named after Aegir, the Scandinavian sea god.

Neptunite forms translucent or opaque black or very dark red prismatic crystals with square cross-sections.

The crystals of Neptunite show piezoelectricity.

Neptunite Gemstones by Colour

This table shows the variety of hues this gemstone can be found in. Click on a photo for more information.
 
General Information
Chemical Formula
KNa
 
2
Li(Fe
2+
 
,Mn
2+
 
)
 
2
Ti
 
2
Si
 
8
O
 
24
Michael O’Donoghue, Gems, Sixth Edition (2006)
Photos of natural/un-cut material from mindat.org
Physical Properties of Neptunite
Mohs Hardness5 to 6
Walter Schumann, Gemstones of the world (2001)
More from other references
Specific Gravity3.19 to 3.23
Walter Schumann, Gemstones of the world (2001)
More from other references
TenacityBrittle
Anthony et al, Handbook of Mineralogy (2001)
Cleavage QualityPerfect
Michael O’Donoghue, Gems, Sixth Edition (2006)
FractureConchoidal
Anthony et al, Handbook of Mineralogy (2001)
Optical Properties of Neptunite
Refractive Index1.692 to 1.734
Michael O’Donoghue, Gems, Sixth Edition (2006) The RI for the alpha, beta and gamma rays is 1.692, 1.702 and 1.734
Optical CharacterBiaxial/+
Michael O’Donoghue, Gems, Sixth Edition (2006)
Birefringence0.029 to 0.045
Michael O’Donoghue, Gems, Sixth Edition (2006)
PleochroismPale yellow - yellow-orange - orange to red-brown
Anthony et al, Handbook of Mineralogy (2001)
DispersionExtreme
Anthony et al, Handbook of Mineralogy (2001)
Colour
Colour (General)Black or very dark red
Michael O’Donoghue, Gems, Sixth Edition (2006)
TransparencyTranslucent,Opaque
Walter Schumann, Gemstones of the world (2001)
More from other references
LustreVitreous
Walter Schumann, Gemstones of the world (2001) Strong vitreous lustreMore from other references
Crystallography of Neptunite
Crystal SystemMonoclinic
Michael O’Donoghue, Gems, Sixth Edition (2006)
HabitPrismatic crystals with square cross-sections
Michael O’Donoghue, Gems, Sixth Edition (2006)
Geological Environment
Where found:Occurs in natrolite veins cutting a glaucophane schist in a serpentinite body
Michael O’Donoghue, Gems, Sixth Edition (2006)
Further Information
Mineral information:Neptunite information at mindat.org
Significant Gem Localities
USA
 
  • California
    • San Benito Co.
      • Diablo Range
        • New Idria Mining District
          • Picacho Peak
Gems, Sixth Edition, Michael O’Donoghue, 2006, p. 433
          • Santa Rita Peak
Wise, W.S & Gill, R.H. (1977): Minerals of the Benitoite Gem mine. Mineralogical Record 8, 442-452
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