Goethite

Goethite is named after Johann Wolfgang von Goethe, celebrated German poet and scientist.

Goethite may form radiating cluster of deep black crystals or massive stalactites that exhibit a rainbow of iridescent colours.

The material is often cut and polished into interesting free forms.
General Information
Chemical Formula
FeO(OH)
Arthur Thomas, Gemstones (2009)
Photos of natural/un-cut material from mindat.org
Physical Properties of Goethite
Mohs Hardness5 to 5.5
Arthur Thomas, Gemstones (2009)
Specific Gravity4.27 to 4.29
Arthur Thomas, Gemstones (2009)
TenacityBrittle
Arthur Thomas, Gemstones (2009)
Cleavage QualityPerfect
Arthur Thomas, Gemstones (2009)
FractureUneven,Splintery
Arthur Thomas, Gemstones (2009)
Optical Properties of Goethite
Refractive Index2.26 to 2.398
Arthur Thomas, Gemstones (2009)
Optical CharacterBiaxial/-
Arthur Thomas, Gemstones (2009)
Birefringence0.138
Arthur Thomas, Gemstones (2009)
PleochroismVisible
Arthur Thomas, Gemstones (2009)
DispersionRelatively strong
Arthur Thomas, Gemstones (2009)
Colour
Colour (General)Black, brown, russet, yellow
Arthur Thomas, Gemstones (2009)
TransparencyOpaque
Arthur Thomas, Gemstones (2009)
LustreAdamantine,Silky,Dull (Earthy),Metallic
Arthur Thomas, Gemstones (2009)
Crystallography of Goethite
Crystal SystemOrthorhombic
Arthur Thomas, Gemstones (2009)
HabitBotryoidal, stalactitic, bladed, columnar
Arthur Thomas, Gemstones (2009)
Further Information
Mineral information:Goethite information at mindat.org
Significant Gem Localities
Nigeria
 
  • Kaduna State
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