How to identify Quartz with dark mineral inclusions

To correctly identify Quartz with dark mineral inclusions (SiO2 with associated mineral), 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

   The quartz matrix is typically colorless, white, or gray, but can also be smoky, amethystine, or citrine. The dark inclusions can be black, dark green, dark brown, or dark gray. The overall appearance is a clear or translucent quartz with distinct dark specks, needles, or masses.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) for the quartz. The luster of the dark mineral inclusions can vary from vitreous to sub-metallic, dull, or earthy, depending on the specific mineral.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Crystalline, often with well-formed quartz crystals. The texture of the inclusions can be acicular (needle-like), platy, granular, or massive within the quartz.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz typically forms hexagonal prisms terminated by hexagonal pyramids. The dark inclusions will exhibit their characteristic crystal habits if well-formed (e.g., prismatic for tourmaline, platy for chlorite, acicular for rutile).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz has no true cleavage, exhibiting conchoidal fracture. The dark mineral inclusions may or may not exhibit cleavage, depending on their mineralogical identity (e.g., amphiboles have two distinct cleavages, tourmaline has poor cleavage).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermal veins, pegmatites, granitic intrusions, metamorphic rocks, and sometimes sedimentary environments. The specific environment is highly dependent on the identity of the dark mineral.

Key Facts

Hardness

7 on Mohs scale for quartz. The hardness of the dark mineral will vary (e.g., 7-7.5 for tourmaline, 2-2.5 for chlorite, 6-6.5 for hematite).

Specific Gravity

2.65 g/cm³ for quartz. The specific gravity of the dark mineral will vary (e.g., 3.0-3.2 for tourmaline, 2.6-3.3 for chlorite, 5.26 for hematite).

Crystal System

Trigonal for quartz. The crystal system of the dark mineral will vary (e.g., trigonal for tourmaline, monoclinic for chlorite, trigonal for hematite).

Color

Colorless to white/gray/smoky quartz with dark (black, dark green, dark brown) inclusions.

Luster

Vitreous for quartz; variable for inclusions (vitreous, sub-metallic, dull, earthy).

Transparency

Transparent to translucent for quartz; opaque to translucent for inclusions.

Fracture

Conchoidal for quartz. The fracture of the dark mineral will vary.

Cleavage

None for quartz. The cleavage of the dark mineral will vary (e.g., poor for tourmaline, perfect basal for chlorite, none for hematite).

Composition

SiO2 (silicon dioxide) for quartz, with the chemical composition of the specific dark mineral.

Physical characteristics

• Crystal Habit: Quartz: Prismatic, often terminated. Inclusions: Variable, can be acicular, prismatic, platy, granular, or massive.
• Cleavage Type: Quartz: None. Inclusions: Variable, depending on the mineral.
• Fracture Type: Quartz: Conchoidal. Inclusions: Variable.
• Tenacity: Quartz: Brittle. Inclusions: Variable (e.g., brittle for tourmaline, flexible for chlorite).
• Luster Type: Quartz: Vitreous. Inclusions: Variable (vitreous, sub-metallic, dull, earthy).

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