Quartz

Quartz species including amethyst, citrine, rose quartz, smoky quartz, rock crystal, and chalcedony varieties with properties and identification.

species/quartz amethyst citrine chalcedony agate

Introduction

Quartz (SiO₂) is the most abundant mineral in Earth's crust and one of the
most versatile gem materials. It occurs in two main forms: macrocrystalline
(visible crystals) and cryptocrystalline (chalcedony varieties).

Despite its commonality, fine quartz specimens—particularly amethyst and
well-coloured citrine—remain popular and valuable.

Mineralogy

Crystal System and Structure

  • Crystal system: Trigonal (low quartz, stable below 573°C)
  • Chemical formula: SiO₂
  • Habit: Hexagonal prisms with pointed terminations
  • Cleavage: None (conchoidal fracture)
  • Twinning: Common (Brazil, Dauphiné, Japan laws)

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⁺)
  • 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

Yellow to orange quartz:

  • Natural citrine: Rare; pale yellow
  • Heat-treated: Most commercial citrine is heated amethyst
  • Colour cause: Fe³⁺ and irradiation-induced colour centres
  • Treated appearance: Often orangey-brown (from heated amethyst)
  • Note: "Madeira citrine" refers to brownish-orange colour

Rose Quartz

Pink quartz:

  • Colour cause: Dumortierite inclusions or Al/P substitution
  • 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
  • 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

Treatments

Quartz undergoes various treatments:

  • Heat treatment: Amethyst → citrine (common, permanent)
  • Irradiation: Colourless → smoky (common)
  • Dyeing: Agate commonly dyed vibrant colours
  • Coating: Surface coatings for iridescence
  • Diffusion: Not applicable to quartz

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

Related Topics

Physical PropertiesOptical PropertiesGemstone Treatments