trigonal Tectosilicates
Amethyst
Formula SiO2:Fe
Crystal Structure
#! Species: Amethyst (Quartz)
#! System: Trigonal (32)
#! Habit: Prismatic with angular colour zoning
trigonal[32]:{10-10}@0.5 + {10-11}@1.2 + {01-11}@1.0[colour_zone:angular] | elongate(c:2.0)
trigonal
32
{10-10} {10-11} {01-11}
Quick Facts
Hardness
7.0
Specific Gravity
2.65
Refractive Index
1.544
Optical Character
Uniaxial +
Physical Properties
- Crystal System
- trigonal
- Hardness (Mohs)
- 7.0
- Specific Gravity
- 2.65
- Cleavage
- None
- Fracture
- Conchoidal
- Lustre
- Vitreous
Optical Properties
- Refractive Index
- 1.544
- Birefringence
- 0.009
- Optical Character
- Uniaxial +
- Dispersion
- 0.013
- Pleochroism
- weak: Purple / Grey-purple (Subtle; best observed in deeply saturated stones)
Characteristics
Colours
PurpleVioletLilacDeep Siberian purple
Localities
BrazilUruguayZambiaRussiaMadagascar
Common Inclusions
Angular colour zoningBrazil-law twinningTwo-phase fluid inclusionsNegative crystalsGoethite/hematiteTiger stripes (colour banding)
Known Treatments
Heat treatment (to citrine)IrradiationCoating
Crystal Forms
prismrhombohedron_positiverhombohedron_negative
Related Minerals
Other minerals in the trigonal system
