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Smoky Quartz is a translucent to opaque, brownish-gray to black variety of crystalline quartz. It is a tectosilicate mineral composed of silicon and oxygen atoms in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Its color ranges from a light, yellowish-brown to a dark, almost black hue, often appearing smoky or cloudy. The intensity of the color can vary within a single crystal. It typically forms as prismatic crystals with pyramidal terminations, often found in geodes, veins, and pegmatites.
How to Identify
- Color
- Ranges from light yellowish-brown to dark brown, grayish-brown, or black. 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 as hexagonal prisms terminated by hexagonal pyramids. Can also be massive or granular.
- Cleavage
- None. Exhibits conchoidal fracture.
- Geological Environment
- Commonly found in granitic pegmatites, hydrothermal veins, alpine-type fissures, and in some metamorphic rocks. It can also occur in alluvial deposits.
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 translucent, rarely opaque
- Fracture: Conchoidal
- Cleavage: None
- Composition: Silicon dioxide (SiO2)
Quick Check
- Color: Brownish-gray to black
- Luster: Vitreous (glassy)
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic crystals, often terminated by rhombohedral faces (forming hexagonal pyramids); also massive, granular, or cryptocrystalline.
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous (glassy)
Formation
Smoky Quartz forms in a similar manner to other macrocrystalline quartz varieties. Its characteristic smoky color is primarily attributed to natural irradiation (gamma rays) of aluminum impurities (Al3+ substituting for Si4+) within the crystal lattice. This irradiation causes a charge transfer, creating color centers that absorb light, resulting in the brown to black coloration. The process can occur naturally over geological timescales in granitic pegmatites, hydrothermal veins, and metamorphic rocks, or it can be induced artificially by exposing clear quartz to radiation. The presence of trace amounts of lithium or hydrogen may also play a role in stabilizing these color centers.
Usage
Smoky Quartz is widely used in jewelry as a gemstone, both faceted and in cabochons. It is also popular in ornamental carvings, decorative objects, and as a collector's mineral specimen. Due to its piezoelectric properties, like other quartz varieties, it has historical and limited modern use in electronic components, though clear quartz is preferred for precision applications. In metaphysical practices, it is often associated with grounding and protection.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, wherever quartz-bearing igneous, metamorphic, or sedimentary rocks occur.
Where to Find
Brazil
Minas Gerais is a major source, producing large, high-quality crystals.
Switzerland
The Swiss Alps are famous for producing fine, often large, smoky quartz crystals from alpine-type fissures.
United States
Notable localities include Colorado (Pikes Peak region), Maine, North Carolina, and Arkansas.
Scotland
The Cairngorm Mountains are historically significant for producing dark smoky quartz, known as Cairngorm.
Madagascar
Produces good quality smoky quartz, often associated with other pegmatite minerals.
Finding Tips
Look in Pegmatites and Hydrothermal Veins
Smoky quartz is frequently found in granitic pegmatites, which are coarse-grained igneous rocks, and in hydrothermal veins, where hot, mineral-rich fluids have deposited minerals in cracks and fissures.
Check Alpine-Type Fissures
In mountainous regions, particularly those with granitic or gneissic bedrock, look for smoky quartz in alpine-type fissures, which are often lined with well-formed crystals.
Examine Alluvial Deposits
Due to its hardness and resistance to weathering, smoky quartz can be found in riverbeds and gravel deposits, often as water-worn pebbles or fragments.
Distinguish from Obsidian
Smoky quartz is crystalline and has a distinct crystal habit (prismatic), while obsidian is amorphous volcanic glass and lacks crystal structure. Smoky quartz will also scratch glass, whereas obsidian will not always.
Distinguish from Black Tourmaline (Schorl)
Schorl typically forms as striated prismatic crystals with a triangular cross-section and has a much more complex chemical composition. Smoky quartz has a hexagonal cross-section and lacks prominent striations on prism faces.
Similar Rocks
Morion
SiO2 (Morion variety of Quartz)
Also known as: Black Quartz
Obsidian
Volcanic glass (amorphous SiO2)
Also known as: Volcanic Glass
Tourmaline (Schorl)
NaFe3Al6(BO3)3Si6O18(OH)4
Also known as: Black Tourmaline
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal (or Hexagonal, often described as such due to its pseudo-hexagonal symmetry)
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide
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