Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Quartz is a tectosilicate mineral 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. When found within igneous or metamorphic host rocks, quartz typically appears as anhedral (irregularly shaped) grains filling interstitial spaces between other minerals, or as euhedral (well-formed) crystals if it crystallized early from a melt or grew in an open cavity. In metamorphic rocks, it often forms interlocking granular textures, particularly in quartzites, or elongated lenses and veins in schists and gneisses. Its presence is a key indicator of the silica saturation of the parent magma or the protolith composition and metamorphic conditions.
How to Identify
- Color
- Typically colorless or white, but can be translucent to opaque and various colors (pink, purple, yellow, brown, black) due to trace impurities or structural defects. In host rocks, it often appears as glassy, colorless to gray grains.
- Luster
- Vitreous (glassy) to greasy.
- Texture
- Granular, interlocking, or anhedral to subhedral crystals within the host rock matrix. Can be massive.
- Crystal Form
- Hexagonal prisms terminated by rhombohedra when euhedral. More commonly anhedral to subhedral grains in host rocks.
- Cleavage
- None, but exhibits conchoidal fracture.
- Geological Environment
- Common in felsic igneous rocks (granite, rhyolite, granodiorite, dacite), and a major constituent of many metamorphic rocks (quartzite, gneiss, schist, granulite). Also found in hydrothermal veins.
Key Facts
- Hardness: 7 on the Mohs scale
- Specific Gravity: 2.65 g/cm³
- Crystal System: Trigonal (often described as hexagonal due to its external symmetry)
- Color: Colorless, white, gray, purple (amethyst), pink (rose quartz), yellow (citrine), brown (smoky quartz), black (morion).
- Luster: Vitreous to greasy
- Transparency: Transparent to translucent to opaque
- Fracture: Conchoidal
- Cleavage: None
- Composition: Silicon dioxide (SiO2)
Quick Check
- Color: Colorless, white, gray, or various other colors due to impurities.
- Luster: Vitreous (glassy) to greasy.
- Streak: White (as it is harder than the streak plate).
Physical Characteristics
- Crystal Habit: Prismatic hexagonal crystals with rhombohedral terminations (euhedral), massive, granular, anhedral to subhedral grains in host rocks.
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous (glassy) to greasy
Formation
Quartz (SiO2) is one of the most abundant minerals in the Earth's crust. In igneous rocks, it crystallizes from silica-rich magmas, typically forming in granites, granodiorites, rhyolites, and dacites. It is a primary mineral in these felsic rocks. In metamorphic rocks, quartz is stable over a wide range of pressure and temperature conditions. It forms through the recrystallization of pre-existing quartz grains (e.g., in quartzites derived from sandstones) or through metamorphic reactions involving silica-bearing minerals. Common metamorphic rocks containing quartz include quartzite, gneiss, schist, and granulite. The presence and morphology of quartz can indicate the metamorphic grade and protolith.
Usage
Quartz-bearing igneous and metamorphic rocks are extensively used as construction materials (e.g., granite for countertops and building facades, quartzite for paving and roofing). Crushed quartz-rich rocks are used as aggregate in concrete and asphalt. High-purity quartz from certain deposits is used in electronics, optics, and ceramics. The silica content is also vital for glass manufacturing. Historically, quartz-rich rocks were used for tools and weapons due to their hardness and conchoidal fracture.
Age Distribution
Ubiquitous across all geological ages where igneous and metamorphic rocks are present, from Precambrian to Cenozoic.
Where to Find
Worldwide
Quartz is one of the most abundant minerals in the Earth's crust and is found in igneous and metamorphic rocks on every continent. Significant occurrences include the Appalachian Mountains (USA), Rocky Mountains (USA), Brazilian Shield, Scandinavian Shield, Himalayas, and various mountain ranges and cratons globally.
Finding Tips
Look for glassy grains
In igneous and metamorphic rocks, quartz often stands out as glassy, colorless to gray grains that lack cleavage and exhibit a conchoidal fracture when broken.
Test hardness
Quartz has a Mohs hardness of 7, meaning it will scratch glass and most common minerals. This is a key diagnostic property.
Observe crystal habit
While often anhedral in host rocks, look for hexagonal crystal forms in vugs or pegmatitic environments within igneous rocks, or elongated lenses in highly deformed metamorphic rocks.
Consider the host rock type
Quartz is a primary component of felsic igneous rocks (e.g., granite) and a major constituent of many metamorphic rocks (e.g., quartzite, gneiss). Its presence helps classify the host rock.
Similar Rocks
Feldspar
(K,Na,Ca)(Al,Si)4O8
Also known as: Aluminosilicate
Calcite
CaCO3
Also known as: Calcium carbonate
Dolomite
CaMg(CO3)2
Also known as: Calcium magnesium carbonate
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide
Explore Quartz in host rock
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.