Sillimanite (Cat's-Eye) Gemstone & Information | Gemopedia by JTV | Gemopedia™
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.
Gray, Green, Brown
Sillimanite-cats-eye classificationCommonNameSillimanite (Cat's-Eye)SpeciesSillimaniteVarietyCat's-Eye SillimaniteColorsGray, Green, BrownAlternateNamesFibroliteGemstoneGroupsKeySeparationsRI, SG, fracture and fibrous appearance.ClassificationCommentsSillimanite is a polymorph of andalusite and kyanite.
Sillimanite-cats-eye chemistry & crystallographyChemicalNamealuminum silicateChemicalFormulaAl2SiO6SynthesisCrystalSystemOrthorhombicChemistryClassificationSilicateNatureNaturalCrystallinityCrystalline to AggregateChemistryComments
Sillimanite-cats-eye optical propertiesTransparencyTranslucent - OpaqueDispersionOpticalCommentsRefractiveIndex1.659-1.68
Tolerance:(+0.004/-0.006)Birefringence0.015-0.021OpticCharacterBiaxialOpticSignPositivePolariscopeReactionAggregate (AGG)FluorescenceSWUV: blue:inert or weak red
LWUV: blue:inert or weak redCCFReactionPleochroismUnobservable
Sillimanite-cats-eye characteristic physical propertiesHardness6-7.5CharacteristicCommentsStreakWhiteSpecificGravity3.14-3.27 Typical:3.25ToughnessPoorInclusionsSillimanite will appear fibrous is often vertically striated and they will contain parallel rutile needles or hypersthene's fibers.LusterVitreousStabilityFractureUnevenCleavagePerfect, in one direction