How to identify Snowflake Obsidian

To correctly identify Snowflake Obsidian (Volcanic Glass (Obsidian) 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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) on fresh surfaces, duller on weathered surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, conchoidal fracture. The 'snowflakes' are slightly rougher to the touch than the glassy matrix.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (glassy) matrix with spherulitic (radial, spherical aggregates) cristobalite inclusions.

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

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

Key Facts

Hardness

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

Specific Gravity

2.35-2.50

Crystal System

Amorphous (glassy) for the obsidian matrix; 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 'snowflakes' are almost pure SiO2 (cristobalite).

Physical characteristics

• Crystal Habit: Amorphous (glassy) matrix; cristobalite forms spherulitic aggregates.
• Cleavage Type: Absent.
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy).

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