How to identify Snowflake Obsidian

To correctly identify Snowflake Obsidian (Volcanic glass 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

   Dark (black, dark gray, sometimes dark brown) with white to grayish-white 'snowflake' patterns.

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

   Aphanitic (glassy) matrix with visible spherulitic inclusions. Smooth to the touch, but can feel slightly granular where spherulites are prominent.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (glassy) matrix; spherulites are microcrystalline radial aggregates of cristobalite.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None (amorphous glass).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Associated with silicic volcanic eruptions, typically found in lava flows, domes, and volcanic necks where rapid cooling of high-silica magma occurred.

Key Facts

Hardness

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

Specific Gravity

2.35-2.60 g/cm³

Crystal System

Amorphous (glass) for the matrix; spherulites are microcrystalline aggregates of cristobalite (tetragonal or orthorhombic at room temperature, but formed as high-temperature cubic polymorph).

Color

Black, dark gray, or dark brown with white to grayish-white spherulites.

Luster

Vitreous (glassy).

Transparency

Opaque to translucent on thin edges.

Fracture

Conchoidal (for the glass matrix).

Cleavage

None.

Composition

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

Physical characteristics

• Crystal Habit: Amorphous (glassy) matrix; spherulites are radial aggregates of microcrystalline cristobalite.
• Cleavage Type: None.
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy).

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