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Smoky Quartz is a translucent to opaque, brownish-gray to black variety of quartz. Its color ranges from a light, barely perceptible brown to a deep, almost black hue (known as Morion). The coloration is a result of natural irradiation of aluminum impurities within the crystal structure. It is a common mineral, often found in well-formed crystals, and is highly valued for its aesthetic appeal.
How to Identify
- Color
- Ranges from light brownish-gray to dark brown, often with a smoky appearance. Can be almost black (Morion). The color is typically uniform but can be zoned.
- Luster
- Vitreous (glassy) on crystal faces and conchoidal fracture surfaces.
- Texture
- Smooth to slightly rough on crystal faces; conchoidal fracture surfaces are very smooth.
- Crystal Form
- Typically forms in hexagonal prisms terminated by rhombohedral faces. Can also be massive or granular.
- Cleavage
- None. Exhibits conchoidal fracture.
- Geological Environment
- Found in igneous rocks (especially granites, pegmatites, and rhyolites), metamorphic rocks (gneisses, schists), and hydrothermal veins. Often associated with other quartz varieties, feldspars, and micas.
Key Facts
- Hardness: 7 on the Mohs scale.
- Specific Gravity: 2.65 g/cm3.
- Crystal System: Trigonal.
- Color: Brownish-gray to black.
- Luster: Vitreous.
- Transparency: Transparent to opaque.
- Fracture: Conchoidal.
- Cleavage: None.
- Composition: Silicon dioxide (SiO2).
Quick Check
- Color: Brownish-gray to black, smoky appearance.
- Luster: Vitreous (glassy).
- Streak: White.
Physical Characteristics
- Crystal Habit: Prismatic, often with hexagonal cross-section, terminated by rhombohedral faces. Also massive, granular.
- Cleavage Type: None.
- Fracture Type: Conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous.
Formation
Smoky Quartz forms in a similar manner to other quartz varieties, primarily through the crystallization of silica-rich fluids. Its characteristic smoky color is attributed to natural irradiation (gamma rays) from nearby radioactive minerals (e.g., granite containing uranium or thorium) acting on trace amounts of aluminum impurities within the crystal lattice. This irradiation causes a displacement of electrons, creating color centers that absorb light, resulting in the smoky hue. It can also form hydrothermally in veins and pegmatites, or as a primary mineral in igneous rocks like granite and rhyolite, and in metamorphic rocks.
Usage
Primarily used as a gemstone, for ornamental carvings, and in lapidary arts. Due to its piezoelectric properties, quartz in general has industrial applications, but smoky quartz is less commonly used for these specific technical applications compared to clear quartz.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, wherever quartz-bearing igneous, metamorphic, or sedimentary rocks occur.
Where to Find
Brazil
Significant producer, particularly from Minas Gerais, known for large, well-formed crystals.
Switzerland
Historically important source, especially in the Alps, producing fine specimens.
United States
Found in various states including Colorado (Pikes Peak region), Maine, North Carolina, and Arkansas.
Scotland
The Cairngorm Mountains are the namesake for 'Cairngorm', a variety of smoky quartz.
Madagascar
Produces good quality smoky quartz, often in large crystals.
Finding Tips
Look in Granitic Pegmatites
These coarse-grained igneous rocks are excellent environments for finding well-formed quartz crystals, including smoky quartz.
Check Hydrothermal Veins
Quartz-rich veins cutting through various rock types can host smoky quartz, often associated with other minerals.
Examine Weathered Outcrops
Smoky quartz is resistant to weathering, so it can be found as float in stream beds or eroded from its host rock.
Utilize UV Light (Caution)
While not a primary identification method for smoky quartz, some quartz varieties can fluoresce. However, the smoky color itself is not typically fluorescent. This is more relevant for other quartz varieties.
Similar Rocks
Obsidian
Volcanic Glass (Amorphous SiO2)
Also known as: Volcanic Glass
Schorl (Black Tourmaline)
Na(Fe2+3)Al6(BO3)3Si6O18(OH)4
Also known as: Black Tourmaline
Andalusite
Al2SiO5
Also known as: Chiastolite (variety)
Scientific Classification
- Mineral Class
- Silicate mineral (Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide
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