How to identify Snowflake Obsidian

To correctly identify Snowflake Obsidian (Volcanic Glass with Cristobalite Inclusions), 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 (cristobalite spherulites).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to sub-vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, conchoidal fracture, with embedded granular or radial spherulitic inclusions.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (glassy) groundmass; cristobalite inclusions are typically spherulitic (radial aggregates of microscopic crystals).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None (due to its amorphous nature).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Formed in areas of explosive felsic volcanism, typically associated with rhyolitic lava flows, domes, and ignimbrite deposits. It occurs in volcanic regions where rapid cooling of silica-rich magma prevents crystallization, followed by subsequent devitrification to form cristobalite spherulites.

Key Facts

Hardness

5-5.5 on Mohs scale (for the glass), cristobalite is slightly harder at 6-7

Specific Gravity

2.35-2.60

Crystal System

Amorphous (glass); Cristobalite is 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 to translucent in thin sections

Fracture

Conchoidal (shell-like)

Cleavage

None

Composition

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

Physical characteristics

• Crystal Habit: Amorphous (glassy) groundmass; cristobalite inclusions are spherulitic (radial aggregates).
• Cleavage Type: None
• Fracture Type: Conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous (glassy)

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