trigonal Tectosilicates

Ametrine

SiO2

Crystal Structure

CDL Notation

 #! 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

Crystal System trigonal

Point Group 32

Crystal Forms (3)

{10-10} {10-11} {0001}

Quick Facts

Hardness

7.0

Specific Gravity

2.65

Refractive Index

1.544

Optical Character

Uniaxial +

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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.