igneous
Mahogany Obsidian
Volcanic Glass (SiO2 with Fe/Mg oxides)
Hardness: 5-5.5 on the Mohs scale. Color: Dark brown to black with reddish-brown streak/patches. Luster: Vitreous to glossy. Fracture: Distinct conchoidal fracture with sharp edges. Amorphous (non-crystalline) structure.
- Hardness
- 5-5
- Color
- Dark brown to black with reddish-brown streak/patches
- Luster
- Vitreous to glossy
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Physical properties
Hardness: 5-5.5 on the Mohs scale. Color: Dark brown to black with reddish-brown streak/patches. Luster: Vitreous to glossy. Fracture: Distinct conchoidal fracture with sharp edges. Amorphous (non-crystalline) structure.
Formation & geological history
Formed when felsic lava cools rapidly with minimal crystal growth. The reddish-brown mahogany patches are caused by inclusions of hematite or magnetite (iron oxides) trapped within the silica glass during cooling.
Uses & applications
Used primarily as an ornamental stone, for carving, cabochon cutting in jewelry, and in knapping to make decorative blades or arrowheads. Historically used for cutting tools due to its razor-sharp conchoidal fracture.
Geological facts
Obsidian is technically a mineraloid rather than a mineral because it lacks a crystalline structure. Its sharp edges can be sharper than high-quality surgical steel scalpels, leading to its specialized modern use in delicate surgeries.
Field identification & locations
Identified by its glass-like vitreous luster, smooth conchoidal fracture surfaces, and characteristic dark brown/black color with reddish hematite swirls. Commonly found in volcanic regions such as Oregon (USA), Mexico, and Iceland.
