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Geosciences LibreTexts

16.53: Peridot

  • Page ID
    5088
  •  

    Peridot
    Chemical composition (Mg,Fe)2SiO4
    Crystal system Orthorhombic
    Habit Prismatic
    Cleavage Poor
    Fracture Conchoidal
    Hardness 6.5
    Optic nature Biaxial +
    Refractive index 1.65 - 1.69
    Birefringence 0.036
    Dispersion Medium, 0.020
    Specific gravity 3.34
    Lustre Vitreous
    Pleochroism Weak

    Figure \(\PageIndex{1}\): Flawless faceted Peridot 
    Photo courtesy of Lembeck Gems

     

    Magnification

     

    Peridot is a Type II stone in the GIA clarity grading system is usually included. Typical inclusions are:

    • disc like liquid and gas inclusions: "lily pads"
    • dark, octahedral chromite crystals

    Figure \(\PageIndex{2}\): Lily Pad inclusions displaying doubling 
    35X Magnification 
    By Barbra Voltaire




     


    The disc-like liquid and gas inclusions, often referred to as lily pads, appear doubled because of the strong birefringence that is a diagnostic characteristic of the gem.