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Included Quartz refers to any quartz crystal (silicon dioxide, SiO2) that contains visible foreign material trapped within its crystalline structure. These inclusions can be solid minerals, liquid (e.g., water, hydrocarbons), or gas (e.g., CO2, methane). The type, size, distribution, and color of the inclusions significantly influence the appearance and value of the quartz. Common mineral inclusions include rutile, tourmaline, chlorite, hematite, goethite, mica, and various amphiboles. Liquid and gas inclusions often appear as bubbles or fluid-filled cavities. Phantom quartz is a specific type where earlier growth stages of the quartz crystal are outlined by layers of included minerals, giving the appearance of 'ghost' crystals within the main crystal.
How to Identify
- Color
- Colorless, white, or any color depending on the quartz variety and the nature of the inclusions. Inclusions themselves can be black (tourmaline), golden (rutile), green (chlorite), red/orange (hematite), etc.
- Luster
- Vitreous (glassy) to greasy on fracture surfaces.
- Texture
- Typically smooth crystal faces, but can be rough or uneven if inclusions protrude or are abundant near the surface.
- Crystal Form
- Hexagonal prisms terminated by rhombohedral faces are common. Inclusions are visible within the crystal.
- Cleavage
- None (lacks cleavage).
- Geological Environment
- Hydrothermal veins, pegmatites, igneous rocks (granites, rhyolites), metamorphic rocks (gneisses, schists), and sedimentary environments (as detrital grains). The specific environment often dictates the type of inclusions present.
Key Facts
- Hardness: 7 on the Mohs scale.
- Specific Gravity: 2.65 g/cm³.
- Crystal System: Trigonal.
- Color: Colorless, white, or various colors depending on inclusions.
- Luster: Vitreous.
- Transparency: Transparent to translucent.
- Fracture: Conchoidal.
- Cleavage: None.
- Composition: Silicon dioxide (SiO2) with various mineral, liquid, or gas inclusions.
Quick Check
- Color: Variable, often clear with colored inclusions.
- Luster: Vitreous.
- Streak: White.
Physical Characteristics
- Crystal Habit: Prismatic, often terminated by rhombohedral faces; massive, granular.
- Cleavage Type: None.
- Fracture Type: Conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous.
Formation
Included quartz forms under a wide range of geological conditions, primarily through hydrothermal processes, magmatic crystallization, and metamorphic recrystallization. The inclusions are typically other minerals, gases, or liquids that become trapped within the growing quartz crystal lattice. The timing of inclusion formation relative to quartz growth is critical: syngenetic inclusions form simultaneously with the quartz, while epigenetic inclusions are introduced later, often along fractures. The specific mineral inclusions depend on the geochemical environment and the availability of other mineral-forming elements during quartz crystallization.
Usage
Primarily used as a gemstone and ornamental stone, particularly varieties with aesthetically pleasing or rare inclusions. Specific types like Rutilated Quartz and Tourmalinated Quartz are highly valued in jewelry. It is also collected by mineral enthusiasts for its aesthetic and scientific interest. Historically, some included quartz varieties were used in folk medicine or spiritual practices.
Age Distribution
Ubiquitous throughout Earth's geological history, from Precambrian to Cenozoic, wherever quartz forms.
Where to Find
Brazil
Renowned for producing a wide variety of included quartz, including rutilated quartz, phantom quartz, and lodolite (garden quartz).
Arkansas, USA
Famous for clear quartz crystals, often containing chlorite or hydrocarbon inclusions.
Madagascar
Known for various quartz types, including those with tourmaline and rutile inclusions.
Pakistan
Produces high-quality quartz with various inclusions, including tourmaline and chlorite.
Switzerland (Alps)
Historical source of fine quartz crystals, sometimes with chlorite or other mineral inclusions.
Finding Tips
Hydrothermal Veins
Look for quartz crystals in hydrothermal vein deposits, especially in areas with granite or metamorphic rocks, as these environments are conducive to the growth of quartz with various mineral inclusions.
Pegmatites
Explore pegmatite dikes, which are known to host large, well-formed quartz crystals and a variety of accessory minerals that can become inclusions.
Mine Dumps and Tailings
Check mine dumps from areas known for quartz or other mineral production, as included quartz specimens may be overlooked or discarded.
Riverbeds and Alluvial Deposits
Water-worn included quartz can be found in riverbeds and alluvial deposits, often exhibiting rounded forms but still showcasing internal features.
Similar Rocks
Chalcedony
Cryptocrystalline Quartz (SiO2)
Also known as: Agate, Jasper, Carnelian, Onyx
Opal
Hydrated Amorphous Silica (SiO2·nH2O)
Also known as: Precious Opal, Common Opal
Scapolite
(Na,Ca)4[(Al,Si)2Si2O8]3(Cl,CO3,SO4)
Also known as: Wernerite
Scientific Classification
- Mineral Class
- Tectosilicate
- Group
- Quartz Group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (with various inclusions)
- Composition
- Silicon dioxide, with trapped foreign materials (minerals, liquids, gases).
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