igneous
Snowflake Obsidian
Spherulitic Volcanic Glass (SiO2 + impurities)
Hardness: 5-5.5 (Mohs scale); Color: Black with white or grey 'snowflake' patterns; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline) glass with crystalline inclusions; Specific gravity: 2.3-2.6.
- Hardness
- 5-5
- Color
- Black with white or grey 'snowflake' patterns
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black with white or grey 'snowflake' patterns; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline) glass with crystalline inclusions; Specific gravity: 2.3-2.6.
Formation & geological history
Formed through the rapid cooling of felsic (silica-rich) lava. The 'snowflakes' are spherulites, which are radial clusters of needle-like cristobalite (a type of quartz) crystals that grew as the glass partially devitrified over time.
Uses & applications
Commonly used in jewelry (cabochons, beads), decorative carvings, and as a popular specimen for rock collectors. Historically, obsidian was used to make sharp cutting tools and blades.
Geological facts
Although it is an igneous rock, it is often called a 'mineraloid' because it lacks a true crystalline structure. The white patches represent the start of a transformation process where the volcanic glass begins to turn into different minerals.
Field identification & locations
Identify in the field by its smooth, glassy black base and conchoidal fracture, distinctively marked by white patches. Major deposits are found in volcanic regions of the United States (Utah, Oregon), Mexico, and Iceland.
