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Quartz is the second most abundant mineral in Earth's continental crust, after feldspar. It is composed of silicon and oxygen atoms in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen atom being shared between two tetrahedra, giving an overall chemical formula of SiO2. It is a defining component of many igneous, metamorphic, and sedimentary rocks. Its hardness and chemical inertness make it highly resistant to weathering, leading to its accumulation in sedimentary environments.
How to Identify
- Color
- Highly variable; colorless, white, gray, purple (amethyst), yellow (citrine), pink (rose quartz), brown (smoky quartz), green, black. Often transparent to translucent.
- Luster
- Vitreous (glassy) to greasy.
- Texture
- Typically smooth to slightly rough on crystal faces; conchoidal fracture surfaces are smooth and curved.
- Crystal Form
- Hexagonal prisms terminated by hexagonal pyramids. Often found as anhedral (irregular) grains in rocks or as massive aggregates. Well-formed crystals are common in vugs and veins.
- Cleavage
- None. Quartz exhibits conchoidal fracture.
- Geological Environment
- Igneous rocks (granite, rhyolite, pegmatite), metamorphic rocks (gneiss, schist, quartzite), sedimentary rocks (sandstone, chert), hydrothermal veins, geodes, and as detrital grains in sediments.
Key Facts
- Hardness: 7 on the Mohs scale
- Specific Gravity: 2.65 g/cm³
- Crystal System: Trigonal (low quartz) or Hexagonal (high quartz)
- Color: Colorless, white, gray, purple, yellow, pink, brown, green, black
- Luster: Vitreous to greasy
- Transparency: Transparent to opaque
- Fracture: Conchoidal
- Cleavage: None
- Composition: Silicon dioxide (SiO2)
Quick Check
- Color: Variable, but often clear, white, or gray.
- Luster: Vitreous (glassy) to greasy.
- Streak: White (even for colored varieties).
Physical Characteristics
- Crystal Habit: Prismatic, massive, granular, cryptocrystalline (e.g., chalcedony). Well-formed crystals are typically hexagonal prisms terminated by rhombohedra (often appearing as hexagonal pyramids).
- Cleavage Type: Absent. Quartz lacks cleavage.
- Fracture Type: Conchoidal (shell-like), producing smooth, curved surfaces with sharp edges.
- Tenacity: Brittle
- Luster Type: Vitreous (glassy) on crystal faces; greasy on massive or fractured surfaces.
Formation
Quartz forms under a wide range of temperatures and pressures. It crystallizes from magmatic melts (e.g., granite, rhyolite), precipitates from hydrothermal solutions in veins and geodes, and recrystallizes during metamorphism (e.g., quartzite). It is also a primary constituent of many sedimentary rocks (e.g., sandstone) due to its resistance to weathering.
Usage
Quartz is one of the most versatile minerals. Industrial uses include glass manufacturing, abrasives, foundry sands, ceramics, electronics (piezoelectric properties), and as a flux in metallurgy. Gemstone varieties (e.g., amethyst, citrine, rose quartz) are widely used in jewelry. It is also a significant component of construction materials.
Age Distribution
Ubiquitous throughout Earth's geological history, from Precambrian to present.
Where to Find
Brazil
World-renowned for large, high-quality quartz crystals, including amethyst and citrine.
Arkansas, USA
Famous for clear, well-formed quartz crystals, often found in veins and vugs.
Alps, Europe
Historical source of fine quartz crystals, including smoky quartz and rock crystal.
Madagascar
Known for various quartz varieties, including rose quartz and amethyst.
Everywhere
As a common rock-forming mineral, quartz is found globally in virtually all geological settings.
Finding Tips
Look for Crystal Forms
In vugs, geodes, or veins, look for characteristic hexagonal prisms with pyramidal terminations. These are often clear or milky white.
Check for Hardness
Quartz is harder than steel (Mohs 7). It will scratch glass and most common minerals. This is a key diagnostic test.
Observe Fracture
Unlike minerals with cleavage, quartz breaks with a distinctive conchoidal (shell-like) fracture, producing smooth, curved surfaces.
Examine Luster
A glassy (vitreous) luster is typical for crystal faces, while massive or fractured surfaces may appear greasy.
Consider Geological Context
Quartz is abundant in granitic rocks, sandstones, and metamorphic rocks like quartzite. Hydrothermal veins are also excellent places to find well-formed crystals.
Similar Rocks
Calcite
Calcite
Also known as: Calcium Carbonate
Feldspar
Feldspar Group
Also known as: Potassium Feldspar, Plagioclase Feldspar
Topaz
Topaz
Also known as: Aluminum Fluorosilicate
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group
- Crystal System
- Trigonal (alpha-quartz, stable below 573°C) or Hexagonal (beta-quartz, stable above 573°C)
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide
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