trigonal Tectosilicates
Ametrine
SiO2
Crystal Structure
#! Species: Ametrine (bicolour Quartz)
#! System: Trigonal (32)
#! Habit: Prismatic hexagonal crystal with amethyst/citrine colour sectors
trigonal[32]:{10-10}@1.0 + {10-11}@0.8 + {0001}@0.3
trigonal
32
{10-10} {10-11} {0001}
Quick Facts
Hardness
7.0
Specific Gravity
2.65
Refractive Index
1.544
Optical Character
Uniaxial +
Physical Properties
Crystal Systemtrigonal
Hardness (Mohs)7.0
Specific Gravity2.65
CleavageNone
FractureConchoidal
LustreVitreous
Optical Properties
Refractive Index1.544
Birefringence0.009
Optical CharacterUniaxial +
Dispersion0.013
Pleochroismweak: Purple / Pale violet to colourless
Colours
Bicolour purple/violet (amethyst sector) and yellow to orange (citrine sector)
Localities
Bolivia (Anahí mine, Santa Cruz province; primary and most celebrated source)Brazil (minor; bicolour quartz from various localities)India (minor occurrences)
Common Inclusions
Two-phase fluid inclusions in both sectorsGrowth zone planes parallel to rhombohedral facesSharp colour sector boundaries (diagnostic of natural bicolour growth)
Known Treatments
Thermal/irradiation treatment of amethyst can produce synthetic ametrine; natural ametrine from Bolivia has specific inclusions distinguishing it from treated material
Crystal Forms
prismrhombohedronpinacoid
Diagnostic Features
Bicolour quartz with distinct amethyst (purple) and citrine (yellow-orange) sectors; sharp colour boundary; uniaxial positive; RI 1.544-1.553; SG 2.65; hardness 7; Bolivia Anahí mine as primary natural source
Notes
Bicolour variety of quartz showing amethyst (purple; Fe4+/Fe3+ colour centre) and citrine (yellow-orange; Fe3+ in different coordination) growth sectors in a single crystal. The Bolivia Anahí mine is documented as the primary commercial source by Sunagawa et al. 1999 (DOI 10.1016/s0022-0248(99)00866-0) [VERIFIED] and Fritsch & Shigley 1994, Gems & Gemology (DOI 10.5741/gems.30.1.4) [VERIFIED]. Crystal system trigonal 32 (same as quartz and amethyst/ citrine). RI and SG from Read 7th ed. (DOI 10.4324/9780080507224) [PARTIALLY_SUPPORTED]. Distinction from heat-treated bicolour quartz: natural ametrine has sharp sector boundaries and specific inclusion suite from the Anahí mine.
Related Minerals
Other minerals in the trigonal system
