How to identify Quartz with dark inclusions

To correctly identify Quartz with dark inclusions (SiO2 (with accessory minerals)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Quartz matrix is typically colorless, white, or smoky. The inclusions are dark, commonly black, dark green, dark brown, or dark red, depending on the accessory mineral.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) on crystal faces and fracture surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Crystalline, often with visible internal inclusions. Can be massive or form distinct euhedral to subhedral crystals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Hexagonal prisms terminated by rhombohedra are common for quartz. Inclusions may show their own characteristic crystal forms (e.g., acicular tourmaline, platy chlorite).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Quartz exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermal veins, pegmatites, granitic rocks, metamorphic rocks, and sedimentary environments (as detrital grains). The specific environment depends on the formation of both the quartz and the included minerals.

Key Facts

Hardness

7 on the Mohs scale (for quartz). The hardness of inclusions varies but will not scratch quartz.

Specific Gravity

2.65 g/cm³ (for pure quartz). Slightly higher if dense inclusions are abundant.

Crystal System

Trigonal (for quartz).

Color

Colorless, white, or smoky quartz with black, dark green, dark brown, or dark red inclusions.

Luster

Vitreous.

Transparency

Transparent to translucent (for quartz matrix). Inclusions are typically opaque.

Fracture

Conchoidal.

Cleavage

None.

Composition

SiO2 (silicon dioxide) with various accessory minerals (e.g., tourmaline, chlorite, hematite, goethite, rutile, sulfides).

Physical characteristics

• Crystal Habit: Prismatic, massive, granular. Inclusions can be acicular, platy, granular, or anhedral.
• Cleavage Type: Absent.
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous.

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