trigonal Oxides
Sapphire
Formula Al2O3:Fe,Ti
Crystal Structure
#! Species: Sapphire (Corundum)
#! System: Trigonal (-3m)
#! Habit: Barrel-shaped with horizontal striations
trigonal[-3m]:{10-10}@1.0[striation:horizontal] + {10-11}@0.7 | elongate(c:1.8) | [silk:dense]
trigonal
-3m
{10-10} {10-11}
Quick Facts
Hardness
9.0
Specific Gravity
3.99
Refractive Index
1.762
Optical Character
Uniaxial -
Physical Properties
- Crystal System
- trigonal
- Hardness (Mohs)
- 9.0
- Specific Gravity
- 3.99
- Cleavage
- None (parting along twin planes)
- Fracture
- Conchoidal to uneven
- Lustre
- Vitreous to adamantine
Optical Properties
- Refractive Index
- 1.762
- Birefringence
- 0.008
- Optical Character
- Uniaxial -
- Dispersion
- 0.018
- Pleochroism
- strong: Violetish-blue / Greenish-blue (Stronger in darker stones; best observed through dichroscope)
- Fluorescence
- Variable; blue often inert; chalky blue-green SWUV in some Sri Lankan
Characteristics
Colours
BlueYellowPinkOrangeGreenPurpleColorlessPadparadscha (pink-orange)
Localities
Kashmir, IndiaMyanmar (Mogok)Sri Lanka (Ceylon)MadagascarAustraliaMontana, USA
Common Inclusions
Rutile silkFingerprintsCrystal inclusions (calcite, apatite, zircon)Hexagonal zoningNegative crystals
Known Treatments
Heat treatmentBeryllium diffusionLead glass fillingFlux healing
Crystal Forms
prismdipyramid
Diagnostic Features
Hexagonal growth zoning; colour banding; silk (rutile needles at 60°); iron absorption bands
Synthetics & Simulants
Known synthetic and simulant versions of Sapphire
Related Minerals
Other minerals in the trigonal system
