Sillimanite Gemstone & Information | Gemopedia by JTV | Gemopedia™

Sillimanite

Beautiful and rare, sillimanite is named for the famous American geologist Benjamin Silliman and was relatively unknown until a substantial find was discovered in Orissa, India in the 1990's. Sillimanite is not only scarce, but also difficult for miners to identify and problematic for cutters. These three attributes ensure that sillimanite remains a true exotic gemstone. Sillimanite ranges from colorless to white, brown, yellow, blue and green in color and consists of compact fibrous material that has a silky luster. A polymorph of kyanite and andalusite, sillimanite makes an exciting addition to any gemstone collection.

Colors

Slate-Blue, Blue-Green, Grayish Green, Violet Blue

  • Sillimanite classification

    CommonName
    Sillimanite
    Species
    Sillimanite
    Variety
    Colors
    Slate-Blue, Blue-Green, Grayish Green, Violet Blue
    AlternateNames
    GemstoneGroups
    KeySeparations
    RI, SG and fracture, possibly fluorescence and birefringence.
    ClassificationComments
    Sillimanite is one of the most difficult gemstones to fashion.
  • Sillimanite chemistry & crystallography

    ChemicalName
    aluminum silicate
    ChemicalFormula
    Al2SiO6
    Synthesis
    CrystalSystem
    Orthorhombic
    ChemistryClassification
    Silicate
    Nature
    Natural
    Crystallinity
    Crystalline
    ChemistryComments
  • Sillimanite optical properties

    Transparency
    Transparent - Translucent
    Dispersion
    Strength: weak fire Value: 0.015
    OpticalComments
    RefractiveIndex
    1.659-1.68
    Tolerance:(+0.004/-0.006)
    Birefringence
    0.015-0.021
    OpticCharacter
    Biaxial
    OpticSign
    Positive
    PolariscopeReaction
    Doubly Refractive (DR)
    Fluorescence
    SWUV: inert or weak red
    LWUV: inert or weak red
    CCFReaction
    Pleochroism
    Trichroic, (in blue gems) strong blue, colorless, and light yellow; (in green gems) strong yellowish green, dark green, and blue
  • Sillimanite characteristic physical properties

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