How to identify Snowflake Obsidian

To correctly identify Snowflake Obsidian (Volcanic Glass (Obsidian) with spherulitic inclusions of cristobalite), 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

   Black to dark gray groundmass with white to grayish-white 'snowflake' patterns.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) on fresh surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, glassy groundmass with slightly rougher, crystalline inclusions.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (glassy) matrix with spherulitic (radial fibrous) inclusions of cristobalite.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None; exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in areas of recent or geologically young felsic volcanic activity, typically associated with rhyolitic lava flows and domes.

Key Facts

Hardness

5-5.5 on the Mohs scale (for the glassy matrix)

Specific Gravity

2.35-2.60

Crystal System

Amorphous (glassy) matrix; Cristobalite inclusions are tetragonal (high-temperature form) or orthorhombic (low-temperature form)

Color

Black to dark gray with white to grayish-white inclusions

Luster

Vitreous (glassy)

Transparency

Opaque

Fracture

Conchoidal

Cleavage

None

Composition

Primarily SiO2 (silica) with minor amounts of Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, and H2O. The white inclusions are composed of cristobalite (SiO2).

Physical characteristics

• Crystal Habit: Amorphous (glassy) for the matrix; spherulitic (radial fibrous aggregates) for the cristobalite inclusions.
• Cleavage Type: None
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle
• Luster Type: Vitreous (glassy)

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