Obsidian

Hardness: 5-5.5 | Color: Typically black or dark grey | Luster: Vitreous/Glassy | Structure: Amorphous (non-crystalline) | Fracture: Conchoidal (shell-like) | Specific Gravity: 2.35-2.60

Formed by the rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. It is relatively young geologically, as it eventually devitrifies into crystals over millions of years.

Historically used for making sharp tools like arrowheads and knives. Today used as a gemstone in jewelry, in high-precision scalpel blades for surgery, and as an ornamental stone.

Obsidian can be so sharp that its edge can reach molecular thinness, making it sharper than high-quality steel scalpels. It is technically a 'mineraloid' because it lacks a crystal structure.

Identified by its distinctive glassy appearance and razor-sharp, curved conchoidal fracture edges. Found in volcanic regions like the American West, Iceland, and Mexico. Collectors look for unique variations like 'snowflake' or 'mahogany' patterns.