How to identify Snowflake Obsidian

To correctly identify Snowflake Obsidian (Volcanic Glass (Obsidian) with Spherulites 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 fractured surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, glassy groundmass with slightly rougher, granular spherulites.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (glassy) groundmass; spherulitic aggregates of microcrystalline 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

   Associated with rhyolitic volcanic flows, domes, and ignimbrite deposits, typically in areas of active or geologically recent volcanism.

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) with spherulitic aggregates of tetragonal cristobalite

Color

Black to dark gray with white to grayish-white inclusions

Luster

Vitreous

Transparency

Opaque to translucent on thin edges

Fracture

Conchoidal

Cleavage

None

Composition

Primarily SiO2 (70-75%), with minor amounts of Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, and H2O. The white 'snowflakes' are cristobalite (SiO2).

Physical characteristics

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

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