Igneous
Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (70% or more silicon dioxide)
Hardness: 5–5.5 Mohs; Color: Typically black but can be brown or green; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal fracture; Specific Gravity: 2.35–2.6
- Hardness
- 5–5
- Color
- Typically black but can be brown or green
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 Mohs; Color: Typically black but can be brown or green; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal fracture; Specific Gravity: 2.35–2.6
Formation & geological history
Formed from rapidly cooling felsic lava where the cooling happens so quickly that crystals do not have time to grow. Most specimens are geologically young (less than few million years old) because it eventually devitrifies into crystalline rock.
Uses & applications
Historically used for razor-sharp tools and weapons. Today used in scalpel blades for delicate surgery, modern jewelry, and decorative items due to its aesthetic appeal.
Geological facts
Obsidian fractures with an edge that is many times sharper than a high-quality steel scalpel. Ancient Aztecs used an obsidian-bladed sword called a Macuahuitl.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture, intense glassy luster, and sharp edges when broken. Commonly found in volcanic regions such as the American Northwest, Mexico, and Iceland.
