Included Quartz

Description

Quartz with dark inclusions refers to specimens of the mineral quartz (silicon dioxide, SiO2) that contain visible dark-colored mineral phases trapped within their crystal structure. The quartz matrix is typically colorless, white, or smoky, allowing the dark inclusions to be clearly observed. The nature of the dark inclusion can vary widely, leading to different appearances and sometimes specific trade names (e.g., 'Phantom Quartz' if the inclusions outline earlier growth stages, 'Garden Quartz' or 'Lodolite' for complex, mossy inclusions). The inclusions can be acicular (needle-like), tabular, granular, or amorphous, and their distribution can be uniform, clustered, or zoned.

How to Identify

Color

Quartz matrix is typically colorless, white, or smoky. Inclusions are dark (black, dark brown, dark green, dark red).

Luster

Vitreous (glassy) for the quartz matrix.

Texture

Smooth to conchoidal fracture for quartz. Inclusions may have their own characteristic textures.

Crystal Form

Hexagonal prisms terminated by rhombohedra for quartz. Inclusions may show their own crystal habits if well-formed (e.g., acicular tourmaline).

Cleavage

None for quartz. Inclusions may exhibit cleavage if they are well-formed crystals.

Geological Environment

Hydrothermal veins, pegmatites, metamorphic rocks, and sometimes sedimentary environments where quartz recrystallizes.

Key Facts

• Hardness: 7 on the Mohs scale (for quartz). The hardness of inclusions may vary but is generally less than quartz.
• Specific Gravity: 2.65 g/cm³ (for pure quartz). May vary slightly depending on the density and proportion of inclusions.
• Crystal System: Trigonal (for quartz).
• Color: Colorless, white, or smoky quartz with black, dark brown, dark green, or dark red inclusions.
• Luster: Vitreous.
• Transparency: Transparent to translucent (for quartz matrix). Inclusions are typically opaque.
• Fracture: Conchoidal (for quartz).
• Cleavage: None (for quartz).
• Composition: SiO2 (quartz) with various dark mineral inclusions (e.g., iron oxides, tourmaline, chlorite, amphiboles, carbonaceous material).

Quick Check

• Color: Clear to smoky quartz with distinct dark internal features.
• Luster: Vitreous.
• Streak: White (for quartz). The streak of the inclusion is usually not observable without crushing.

Physical Characteristics

• Crystal Habit: Prismatic, often terminated by rhombohedra. Inclusions can be acicular, tabular, granular, or amorphous within the quartz.
• Cleavage Type: Absent (for quartz).
• Fracture Type: Conchoidal (for quartz).
• Tenacity: Brittle (for quartz).
• Luster Type: Vitreous.

Formation

Quartz with dark inclusions forms when other mineral phases are trapped within the growing quartz crystal. This can occur during hydrothermal activity, pegmatite formation, or metamorphic processes. The specific dark mineral inclusion depends on the local geological environment and the available elements during crystallization. Common dark inclusions include various iron oxides (e.g., hematite, goethite), tourmaline (e.g., schorl), chlorite, amphiboles, and sometimes carbonaceous material.

Usage

Primarily used as a gemstone or ornamental stone, particularly specimens with aesthetically pleasing or distinct inclusions. Also collected by mineral enthusiasts. The quartz itself has widespread industrial uses, but included varieties are typically valued for their aesthetic appeal.

Age Distribution

Formed throughout geological history, from Precambrian to Cenozoic, wherever quartz crystallization occurs.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicate

Group

Tectosilicate (Quartz Group)

Crystal System

Trigonal

Chemical Formula

SiO2 (with various dark mineral inclusions)

Composition

Silicon dioxide with varying amounts of other dark mineral phases.

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