How to identify Snowflake Obsidian

To correctly identify Snowflake Obsidian (Volcanic glass 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 with white to light gray 'snowflake' patterns.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) for the obsidian matrix; dull to sub-vitreous for the cristobalite inclusions.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, glassy matrix with slightly rougher, granular inclusions. Amorphous overall.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (glassy) matrix; the cristobalite forms radial, spherical aggregates (spherulites) within the glass.

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 or domes where rapid cooling occurred, followed by partial devitrification.

Key Facts

Hardness

5-5.5 on Mohs scale (for the obsidian matrix); cristobalite is slightly harder, around 6-7.

Specific Gravity

2.35-2.60 g/cm³

Crystal System

Amorphous (obsidian matrix); Tetragonal (cristobalite inclusions, though often cryptocrystalline within spherulites)

Color

Black to dark gray with white to light gray inclusions

Luster

Vitreous (glassy)

Transparency

Opaque to translucent on thin edges

Fracture

Conchoidal

Cleavage

None

Composition

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

Physical characteristics

• Crystal Habit: Amorphous (glassy) matrix with spherulitic (radial, spherical) aggregates of cristobalite.
• Cleavage Type: None
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle
• Luster Type: Vitreous (glassy) for the obsidian; dull to sub-vitreous for the cristobalite.

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