Sillimanite (Cat's-Eye) Gemstone & Information | Gemopedia by JTV | Gemopedia™

Sillimanite (Cat's-Eye)

The term cat's eye, or chatoyancy, is used to describe a phenomenal optical property in gemstones, in this case, sillimanite. The effect, when present, appears as a bright, narrow slit similar to the pupils in the eyes of your favorite feline. This phenomenon is caused by parallel fibrous or needle-like inclusions that interfere with the passage of light throughout the crystal, scattering and reflecting light back to the viewer as a thin line. Best observed when gems are cut en cabochon, cat's eye sillimanite makes a phenomenal addition to any gemstone collection.

Colors

Gray, Green, Brown

  • Sillimanite-cats-eye classification

    CommonName
    Sillimanite (Cat's-Eye)
    Species
    Sillimanite
    Variety
    Cat's-Eye Sillimanite
    Colors
    Gray, Green, Brown
    AlternateNames
    Fibrolite
    GemstoneGroups
    KeySeparations
    RI, SG, fracture and fibrous appearance.
    ClassificationComments
    Sillimanite is a polymorph of andalusite and kyanite.
  • Sillimanite-cats-eye chemistry & crystallography

    ChemicalName
    aluminum silicate
    ChemicalFormula
    Al2SiO6
    Synthesis
    CrystalSystem
    Orthorhombic
    ChemistryClassification
    Silicate
    Nature
    Natural
    Crystallinity
    Crystalline to Aggregate
    ChemistryComments
  • Sillimanite-cats-eye optical properties

    Transparency
    Translucent - Opaque
    Dispersion
    OpticalComments
    RefractiveIndex
    1.659-1.68
    Tolerance:(+0.004/-0.006)
    Birefringence
    0.015-0.021
    OpticCharacter
    Biaxial
    OpticSign
    Positive
    PolariscopeReaction
    Aggregate (AGG)
    Fluorescence
    SWUV: blue:inert or weak red
    LWUV: blue:inert or weak red
    CCFReaction
    Pleochroism
    Unobservable
  • Sillimanite-cats-eye characteristic physical properties

    Hardness
    6-7.5
    CharacteristicComments
    Streak
    White
    SpecificGravity
    3.14-3.27 Typical:3.25
    Toughness
    Poor
    Inclusions
    Sillimanite will appear fibrous is often vertically striated and they will contain parallel rutile needles or hypersthene's fibers.
    Luster
    Vitreous
    Stability
    Fracture
    Uneven
    Cleavage
    Perfect, in one direction