Obsidians

Obsidian is a naturally occurring volcanic glass, commonly referred to in the plural as obsidians. It forms when felsic lava erupts or extrudes at the surface and rapidly quenches, preventing crystal growth. This rapid cooling yields a smooth, glassy texture and a sharp edge when fractured. Technically, obsidian is not a mineral but a mineraloid composed mainly of silicon dioxide (SiO2) with trace elements that give color.

Obsidian typically appears black or dark in color, but can be green, brown, or red (mahogany) depending on impurities and vesicle content. It exhibits conchoidal fracture, which creates very sharp edges and makes it highly valued for tool-making. Its hardness is about 5–5.5 on the Mohs scale, and its density is around 2.4–2.6 g/cm3.

Geologically, obsidian forms in silicic lava flows and near volcanic domes where lava erupted and cooled rapidly at or near the surface. It is found in many volcanic regions around the world, including the Americas, Europe, and Asia, often as accessory components of rhyolitic and andesitic deposits.

In archaeology, obsidian has long been used to produce cutting tools, blades, and scrapers, and its trade networks helped transport valuable glass across regions. Today it remains used by lapidaries and for decorative objects. Obsidian hydration dating is a common technique for estimating the age of obsidian artifacts by measuring the thickness of the water-rich layer that forms on freshly exposed surfaces over time.

Despite its usefulness, obsidian is brittle and can shatter if struck or dropped.