Quartz
Quartz species including amethyst, citrine, rose quartz, smoky quartz, rock crystal, and chalcedony varieties with properties and identification.
Introduction
Quartz (SiO₂) is a silicon dioxide mineral crystallising in the trigonal system, the
most abundant mineral in Earth's crust and the most versatile gem species. It occurs in
two structural forms: macrocrystalline (amethyst, citrine, smoky quartz, rose quartz,
rock crystal) and cryptocrystalline chalcedony (agate, carnelian, chrysoprase, onyx,
jasper). All share RI 1.544–1.553 (uniaxial positive), birefringence 0.009, SG 2.65,
and hardness 7 Mohs; these consistent values make quartz one of the easiest minerals to
identify. [1] Purple amethyst owes its colour to iron plus irradiation-induced
colour centres (Fe³⁺) [2]; most commercial citrine is
heat-treated amethyst, producing the typical orangey-brown rather than natural citrine's
pale yellow. The Anahí mine in Bolivia is the sole commercial source of ametrine
(bicoloured amethyst-citrine in one crystal). Tiger's eye from South Africa shows chatoyancy
from pseudomorphic crocidolite replacement. [3] Chalcedony
varieties are dyed routinely; agate absorbs dye readily, requiring visual examination
and spectroscopic confirmation when authenticity matters.
Mineralogy
Crystal System and Structure
Physical Properties
| Property | Value |
|---|---|
| Hardness | 7 Mohs |
| Specific gravity | 2.65 |
| Refractive index | 1.544–1.553 |
| Birefringence | 0.009 |
| Optic character | Uniaxial positive |
| Pleochroism | Weak (in coloured varieties) |
| Lustre | Vitreous |
Macrocrystalline Quartz
Amethyst
Purple to violet quartz:
- Colour cause: Iron + irradiation (Fe³⁺ → Fe⁴⁺) [2]
- Range: Pale lilac to deep purple
- Best colour: Deep "Siberian" purple with red flashes
- Sources: Brazil (major), Uruguay, Zambia, Madagascar
- Zoning: Often shows angular colour zoning
Citrine
Rose Quartz
Pink quartz:
- Colour cause: Dumortierite inclusions or Al/P substitution [2]
- Character: Usually translucent, rarely transparent
- Asterism: Rare star rose quartz exists
- Sources: Brazil, Madagascar, South Dakota
Smoky Quartz
Brown to black quartz:
- Colour cause: Aluminium + natural irradiation [2]
- Range: Pale brown ("cairngorm") to nearly black ("morion")
- Artificial: Can be created by irradiating colourless quartz
- Sources: Worldwide; Scotland (historic), Brazil, Switzerland
Other Varieties
| Variety | Description |
|---|---|
| Rock crystal | Colourless; pure SiO₂ |
| Ametrine | Bicoloured amethyst-citrine; Bolivia |
| Prasiolite | Green quartz (usually heated amethyst) |
| Milky quartz | White, translucent; fluid inclusions |
| Rutilated quartz | Golden rutile needle inclusions |
| Tourmalinated quartz | Black tourmaline inclusions |
Ametrine
Chalcedony Varieties
Cryptocrystalline quartz (fine-grained, often translucent):
Plain Chalcedony
- Appearance: Translucent, waxy lustre
- Colours: Grey, blue, white, pale colours
- Blue chalcedony: Prized; Turkey, Namibia
- Chrysoprase: Apple-green (nickel-coloured); finest chalcedony
Agate
Banded chalcedony:
- Characteristic: Curved, concentric bands
- Colours: Any colour; often dyed
- Types: Moss agate, dendritic agate, fire agate
- Sources: Brazil, Uruguay, India, Madagascar
Carnelian and Sard
Orange to reddish-brown chalcedony:
- Carnelian: Orange to red-orange
- Sard: Brownish-red (darker than carnelian)
- Colour cause: Iron oxide
- Treatment: Often heat-enhanced
Onyx and Sardonyx
- Onyx: Black and white banded chalcedony
- Sardonyx: Sard (reddish-brown) with white bands
- Note: Much "black onyx" is dyed grey chalcedony
- Uses: Cameos, intaglios (carved gems)
Jasper
Opaque, fine-grained quartz:
- Character: Opaque with various colours and patterns
- Types: Red jasper, picture jasper, bloodstone
- Bloodstone: Dark green with red spots (heliotrope)
- Uses: Carvings, cabochons, decorative objects
Inclusions and Features
Quartz hosts many interesting inclusions:
Common Inclusions
| Inclusion | Appearance | Creates |
|---|---|---|
| Rutile needles | Golden, needle-like | Rutilated quartz |
| Tourmaline | Black rods | Tourmalinated quartz |
| Goethite/hematite | Red-brown plates | Strawberry quartz |
| Dumortierite | Blue-violet fibres | Some rose quartz colour |
| Fluid inclusions | Two-phase negative crystals | Common in all quartz |
Special Effects
- Asterism: Star effect in rose quartz (rare)
- Chatoyancy: Cat's eye quartz (sillimanite inclusions)
- Tiger's eye: Chatoyant from replaced crocidolite [3]
Treatments
Synthetic Quartz
Identification Summary
Key features for quartz identification:
- RI: 1.544–1.553 (distinctive)
- SG: 2.65 (constant)
- Birefringence: 0.009
- Hardness: 7 (scratches glass easily)
- No cleavage: Conchoidal fracture
- Bull's eye: Interference figure with cross and rings
References
- ↑ 1. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.
- ↑ 2. Fritsch, E.; Rossman, G. (1988). An Update on Color in Gems. Part 2: Colors Involving Multiple Atoms and Color Centers. Gems & Gemology, 24(1), 3–15. DOI: 10.5741/gems.24.1.3.
- ↑ 3. Fritsch, E.; Rossman, G. (1988). An Update on Color in Gems. Part 3: Colors Caused By Band Gaps and Physical Phenomena. Gems & Gemology, 24(2), 81–102. DOI: 10.5741/gems.24.2.81.