Names

Quartz

Intro Quartz is the definitive low-temperature trigonal polymorph of silicon dioxide (SiO₂), stable up to 573 °C at ambient pressures. As the defining benchmark for Mohs hardness 7 and one of the most abundant rock-forming framework tectosilicates in the Earth's continental crust, quartz is renowned for its strong piezoelectricity, chemical durability, absence of cleavage, and extensive suite of colored macrocrystalline and microcrystalline (chalcedony) varieties.

Basic Information Show Hide

Mineral ID

quartz

Common Synonyms / Aliases

Alpha-quartz, Low-quartz, Rock crystal (macro-crystalline colorless), Silica, Quarz

Chemistry Show Hide

Chemical Formula

SiO₂

Elements

Silicon (Si), Oxygen (O)

Minor Elements / Impurities

Fe, Al, Ti, Mn, Li, Na, K, H₂O

Empirical Formula

SiO₂

Molecular Weight (g/mol)

60.08

Solubility

Insoluble in water and standard organic solvents; very slightly soluble in pure water under supercritical hydrothermal conditions; dissolved by aqueous hydrofluoric acid (HF) and hot concentrated alkali metal hydroxide solutions (KOH, NaOH).

Reaction to Acid

None (Completely inert to hydrochloric (HCl), nitric (HNO₃), and sulfuric (H₂SO₄) acids at ambient and boiling temperatures; reacts exclusively with hydrofluoric acid (HF)).

Classification Show Hide

Mineral Type

Mineral Species

IMA Status

Grandfathered

IMA Symbol

Qtz

Strunz Classification

04.DA.05

Dana Classification

75.01.03.01

Mineral Group

Quartz Group (Silicates / Oxides)

Crystal Structure Show Hide

Crystal System

Trigonal (low/alpha quartz); hexagonal symmetry for high/beta quartz above 573 °C

Crystal Habit

Prismatic six-sided crystals with pyramids, horizontal striations, drusy coatings, massive aggregates, microcrystalline chalcedony, and geode fillings.

Crystal Class

Trigonal trapezohedral, 32 (P3₁21 or P3₂21)

Space Group

P3₁21 or P3₂21

Cell Parameter A (Angstrom)

4.913

Cell Parameter B (Angstrom)

4.913

Cell Parameter C (Angstrom)

5.405

Cell Angle Alpha (Degrees)

Cell Angle Beta (Degrees)

Cell Angle Gamma (Degrees)

120

Unit Cell Z

Crystal Twinning

Exceptionally common; includes Dauphiné twins (penetration twin with c-axis as twin axis), Brazil twins (optical contact twin on {11-20}), Japan Law twins (contact twin on {11-22} at an angle of 84°33'), and Esterel Law twins.

Common Crystal Faces

{10-10} prism, {10-11} positive rhombohedron, {01-11} negative rhombohedron

Physical Properties Show Hide

Primary Color

ColorlessPurple (P)PinkYellow (Y)Green (G)WhiteGrayBrownBlack

Common Colors

Colorless, white, gray, purple, yellow, pink, brown, black, and green

Streak

White

Luster

Vitreous (glassy); greasy on fracture surfaces; waxy to dull in microcrystalline forms

Transparency / Diaphaneity

Transparent to translucent (opaque in massive, heavily included, or cryptocrystalline varieties).

Hardness (Mohs)

Cleavage

Poor

Cleavage Direction

Indistinct on {0111} and {1011}

Cleavage Angle / Parting Quality

No true cleavage; indistinct parting may occur on Brazil-law twin planes.

Fracture

Conchoidal to sub-conchoidal.

Tenacity

Brittle

Specific Gravity

2.65 - 2.66

Magnetism

Non-magnetic (diamagnetic).

Melting Point

1713 °C

Thermal Stability

Undergoes a reversible, displacive phase transition to high-temperature hexagonal beta-quartz (β-SiO₂) at 573 °C (1063 °F) at 1 atm, accompanied by structural volume expansion; converts to tridymite above 870 °C and cristobalite above 1470 °C.

Optical Properties Show Hide

Refractive Index

1.544 - 1.553

Refractive Index Alpha

1.544

Refractive Index Beta

1.544

Refractive Index Gamma

1.553

Optical Axial Angle (2V)

0° (uniaxial)

Optical Sign / Extinction

Positive (+)

Birefringence

0.009

Dispersion

0.013

Optical Character

Uniaxial+

Pleochroism

None to weak (in colored macrocrystalline varieties such as Amethyst, Citrine, and Smoky Quartz).

Pleochroism Details

Weak dichroism in dark amethyst (purple / reddish-purple) and smoky quartz (yellowish-brown / reddish-brown); none in pure colorless rock crystal.

Optical Phenomena

Asterism (6-rayed star caused by oriented rutile inclusions), chatoyancy (cat's-eye quartz, tiger's eye), iridescence (iris quartz / rainbow quartz due to thin-film fracture interference), optical circular polarization (rotates plane-polarized light along the c-axis).

Fluorescence

Inert to weakly fluorescent under UV radiation; diagnostic cathode-luminescence (CL) colors reveal growth zoning (blue, red, or greenish-yellow depending on [AlO₄] and structural defect centers).

Luminescence / Phosphorescence / Thermoluminescence

Generally non-fluorescent; some specific varieties or included specimens display weak green, yellow, or blue fluorescence under SW-UV (254 nm) or LW-UV (365 nm); strongly triboluminescent (emits flashes of light when crystals are struck or rubbed together in darkness).

Absorption Spectrum

Highly transparent across UV, visible, and near-IR regions; diagnostic vibrational infrared absorption bands at 1080 cm⁻¹, 798 cm⁻¹, 778 cm⁻¹ (Si-O-Si stretching doublet), and 460 cm⁻¹ (Si-O bending).

Occurrence and Formation Show Hide

Formation Process

Crystallizes in igneous magmas and pegmatites, hydrothermal veins, metamorphic quartzites, and sedimentary environments.

Geological Environment

Igneous, metamorphic, and sedimentary settings including quartz veins, geodes, pegmatites, and detrital sands.

Associated Minerals

Feldspar, Muscovite, Biotite, Tourmaline, Calcite, Fluorite, Pyrite, Hematite

Type Locality

No single type locality; quartz was recognized historically and is grandfathered

Major Producing Countries / Localities

AustraliaAustriaBrazilCanadaChinaFranceGermanyIndiaItalyJapanMadagascarMexicoNamibiaNorwayPakistanPeruRussiaSouth AfricaSpainSwitzerlandUnited KingdomUnited States

Economic Deposit Type

Hydrothermal veins, granite pegmatites, placer deposits, quartzites, and silica-sand beds

History and Naming Show Hide

Discovery Year

Ancient / Prehistoric (Documented in classical Greek mineral treatises by Theophrastus in ~300 BCE as κρύσταλλος (krýstallos)).

Name Origin

Derived from the German word Quarz, of Middle High German / Slavic origin (from Polish kwardy, meaning "hard").

Type Specimen Repository

None designated; grandfathered mineral

Safety and Related Information Show Hide

Wearability

Excellent

Health Hazards

Completely non-toxic and inert in solid hand specimen and jewelry forms; long-term occupational inhalation of respirable crystalline silica (RCS) dust generated during dry cutting, crushing, or mining operations presents severe health hazards (causes silicosis and classified as an IARC Group 1 human carcinogen).

Handling Precautions

Safe for ultrasonic, steam, and chemical cleaning; always enforce wet-cutting and wear certified N95/FFP3 particulate respirators when cutting, grinding, or polishing bulk quartz specimens.

Varieties

Rock Crystal, Amethyst, Citrine, Smoky Quartz, Rose Quartz, Prasiolite, Ametrine, Milky Quartz, Rutilated Quartz, Tourmalinated Quartz, Morion, Chalcedony (cryptocrystalline varieties: Agate, Jasper, Carnelian, Onyx, Chrysoprase).

Polytypes

None (Polymorphs include Beta-quartz, Tridymite, Cristobalite, Coesite, Stishovite, Moganite, Seifertite).

Related Minerals

Tridymite (SiO₂), Cristobalite (SiO₂), Coesite (SiO₂), Stishovite (SiO₂), Moganite (SiO₂), Opal (SiO₂·nH₂O), Feldspar group.

Commercial and Gemology Show Hide

Common Treatments

Colored and commercial quartz may be heat-treated, irradiated, dyed, coated, or fracture-filled; natural rock crystal is commonly untreated.

Enhancements

Irradiation can produce smoky quartz; heat can modify amethyst or citrine; coatings and dyeing are used for decorative material.

Common Cuts

Brilliant cutstep/emerald cutfantasy bespoke facetingCabochonsBeadsspherical spheresintricate lapidary carvingsoptical waveplatesand oscillator plates.

Rarity / Market Grade

Extremely Common (High-clarity inclusion-free giant crystals, aesthetic museum-grade clusters, and rare natural unheated color phases like natural ametrine or prasiolite are Rare to Museum-grade).