Obsidian

Obsidian is a naturally occurring volcanic glass formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is not a true mineral because it is not crystalline and its composition is too complex and variable. It is famous for its jet-black appearance and its ability to produce extremely sharp edges, which led to its use as tools and weapons since prehistoric times.

Formation & Geology

Obsidian forms when highly viscous lava cools so quickly that atoms are unable to arrange themselves into a structured crystalline lattice. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows. Because of its lack of crystal structure, obsidian is metastable and over millions of years, it eventually devitrifies into fine-grained quartz crystals.

Physical Characteristics

Obsidian is typically jet black, although it can be dark green, brown, or even gray. Trace elements or inclusions can create varieties like 'Snowflake Obsidian' (white cristobalite inclusions), 'Rainbow Obsidian' (iridescent bands), or 'Gold Sheen Obsidian'. It has a Mohs hardness of 5 to 6 and is relatively brittle. Its most famous characteristic is its smooth, conchoidal fracture which results in edges as sharp as a molecule.

Historical Significance

Obsidian was a vital resource for stone-age cultures. It was used to create arrowheads, spear points, and knives that were sharper than any steel. In ancient Mesoamerica, the Aztecs created the 'macuahuitl', a wooden club embedded with obsidian blades. In modern times, obsidian blades are still used in some specialized surgical scalpels because they can produce a much finer cut than steel, leading to faster healing and less scarring.