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Quartz with inclusions refers to any variety of quartz (silicon dioxide, SiO2) that contains visible foreign mineral matter, fluid, or gas trapped within its crystal structure. These inclusions can be microscopic or macroscopic, and their type, size, and distribution significantly influence the appearance and value of the quartz. The host quartz itself can be clear, milky, smoky, or amethystine. The presence of inclusions provides a unique record of the geological environment in which the quartz formed.
How to Identify
- Color
- Variable, depending on the host quartz and the inclusions. Host quartz can be colorless, white, purple, brown, or black. Inclusions can be metallic (e.g., rutile, tourmaline), green (e.g., chlorite, actinolite), red (e.g., hematite), or other colors.
- Luster
- Vitreous (glassy) for the quartz host. Inclusions may have their own characteristic luster (e.g., metallic for rutile, silky for asbestos minerals).
- Texture
- Typically smooth, conchoidal fracture for the quartz. The texture may be interrupted or enhanced by the presence of inclusions, which can appear as needles, plates, dendrites, or amorphous masses within the quartz.
- Crystal Form
- Hexagonal prisms terminated by rhombohedra are common for quartz. Inclusions often follow the growth patterns of the quartz crystal (e.g., phantom growths) or appear as randomly oriented needles or fibers.
- Cleavage
- None (quartz). Inclusions may exhibit their own cleavage, but it is typically not observable within the quartz host.
- Geological Environment
- Hydrothermal veins, pegmatites, metamorphic rocks (schists, gneisses), sedimentary geodes, and alluvial deposits. The specific environment dictates the types of inclusions present.
Key Facts
- Hardness: 7 on the Mohs scale (for quartz host). Inclusions may have different hardnesses.
- Specific Gravity: 2.65 g/cm³ (for pure quartz). May vary slightly depending on the density and abundance of inclusions.
- Crystal System: Trigonal (for quartz host).
- Color: Highly variable, depending on the host quartz and the specific inclusions (e.g., colorless with golden rutile, clear with green chlorite, smoky with black tourmaline).
- Luster: Vitreous.
- Transparency: Transparent to translucent.
- Fracture: Conchoidal.
- Cleavage: None.
- Composition: SiO2 (silicon dioxide) with various mineral, fluid, or gas inclusions.
Quick Check
- Color: Variable, often colorless to milky with internal colored features.
- Luster: Vitreous (glassy).
- Streak: White.
Physical Characteristics
- Crystal Habit: Prismatic, often terminated by rhombohedra. Inclusions can be acicular, fibrous, platy, dendritic, or amorphous within the quartz.
- Cleavage Type: None.
- Fracture Type: Conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous.
Formation
Quartz with inclusions forms under a wide range of geological conditions where quartz crystallizes in the presence of other minerals. The inclusions are typically pre-existing minerals that are engulfed by the growing quartz crystal, or minerals that co-precipitate with quartz. This can occur in igneous environments (e.g., pegmatites, hydrothermal veins), metamorphic environments (e.g., schists, gneisses), and sedimentary environments (e.g., geodes, concretions). The specific type of inclusion depends on the local geochemistry and pressure-temperature conditions during quartz formation.
Usage
Primarily used as ornamental stones, in jewelry, and for metaphysical purposes. Specific types like rutilated quartz are highly prized. Some included quartz, particularly those with fluid inclusions, are studied in academic research to understand paleofluid compositions and geological processes.
Age Distribution
Ubiquitous throughout geological time, from Precambrian to Cenozoic.
Where to Find
Brazil
Renowned for producing a wide variety of included quartz, including rutilated quartz, tourmalinated quartz, and lodolite (garden quartz).
Madagascar
Known for high-quality rutilated quartz and other included varieties.
Arkansas, USA
Produces clear quartz with various inclusions, including chlorite and sometimes hydrocarbons.
Switzerland (Alps)
Historically significant for quartz with various alpine inclusions, including chlorite and actinolite.
Pakistan
Source of high-quality rutilated quartz and other included varieties.
Finding Tips
Look for Transparency
Included quartz is often found in areas known for clear quartz. Look for transparent to translucent quartz crystals that allow for visibility of internal features.
Examine Hydrothermal Veins and Pegmatites
These geological settings are prime locations for the formation of quartz with various mineral inclusions due to the presence of diverse mineralizing fluids.
Check Alluvial Deposits
Riverbeds and gravel deposits can contain weathered-out quartz crystals, some of which may contain inclusions.
Use a Loupe or Magnifier
Many inclusions are small and require magnification to be properly identified and appreciated.
Similar Rocks
Clear Quartz
SiO2
Also known as: Rock Crystal
Amethyst
SiO2 (with Fe impurities)
Also known as: Purple Quartz
Smoky Quartz
SiO2 (with natural irradiation)
Also known as: Morion
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (with various mineral inclusions)
- Composition
- Silicon dioxide with trapped foreign matter. Common inclusions include: Rutile (TiO2), Tourmaline (complex borosilicate), Chlorite (complex phyllosilicate), Actinolite (Ca2(Mg,Fe)5Si8O22(OH)2), Hematite (Fe2O3), Goethite (FeO(OH)), Pyrite (FeS2), Mica (phyllosilicates), various fluid inclusions (water, CO2, hydrocarbons), and gas inclusions.
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