Sillimanite Gemstone & Information | Gemopedia by JTV | Gemopedia™
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.
Slate-Blue, Blue-Green, Grayish Green, Violet Blue
Sillimanite classificationCommonNameSillimaniteSpeciesSillimaniteVarietyColorsSlate-Blue, Blue-Green, Grayish Green, Violet BlueAlternateNamesGemstoneGroupsKeySeparationsRI, SG and fracture, possibly fluorescence and birefringence.ClassificationCommentsSillimanite is one of the most difficult gemstones to fashion.
Sillimanite chemistry & crystallographyChemicalNamealuminum silicateChemicalFormulaAl2SiO6SynthesisCrystalSystemOrthorhombicChemistryClassificationSilicateNatureNaturalCrystallinityCrystallineChemistryComments
Sillimanite optical propertiesTransparencyTransparent - TranslucentDispersionStrength: weak fire Value: 0.015OpticalCommentsRefractiveIndex1.659-1.68
Tolerance:(+0.004/-0.006)Birefringence0.015-0.021OpticCharacterBiaxialOpticSignPositivePolariscopeReactionDoubly Refractive (DR)FluorescenceSWUV: inert or weak red
LWUV: inert or weak redCCFReactionPleochroismTrichroic, (in blue gems) strong blue, colorless, and light yellow; (in green gems) strong yellowish green, dark green, and blue
Sillimanite characteristic physical propertiesHardness6-7.5CharacteristicCommentsStreakWhiteSpecificGravity3.14-3.27 Typical:3.25ToughnessPoorInclusionsSillimanite is often vertically striated and they might contain rutile needles or hypersthene's fibers.LusterVitreousStabilityFractureUnevenCleavagePerfect, in one direction