#conchoidal fracture

Norfolk Trotter - Conchoidal Fracture

Staticism International

Why is obsidian so useful for tools?

The ancient inhabitants of many continents knew the properties of obsidian. This black volcanic glass was a key component in tools and hunting weapons; arrowheads and shards from their production are found all over the world and trading paths between different civilizations can even be tracked using obsidian.

The reason why this rock was so useful comes from the structure of the rock. Obsidian isn’t a mineral, by definition. Minerals have a defined structure that repeats over and over again. Obsidian is what we’d call a “glass”. A mineral growing from lava needs time to grow; atoms need time to move together and form a defined structure. If lava cools off too quickly, it can instead have all its atoms locked into whatever format sat there when the magma was molten, a state we call a glass.

A glass has no defined, long-term structure, so it doesn’t break into crystal faces. This property means glasses are strong in all directions and when broken they will have what we call “conchoidal fractures”. This is different from crystals; they tend to break along fracture or “cleavage” plains controlled by the arrangement of the atoms. You can see the remnant of those fractures in the rippled breaks at the edge of this stone tool artifact; the fractures formed at a single point and widened as they broke outwards.

A skilled worker using obsidian can create a series of conchoidal fractures around the edge that bring the rock to an extremely sharp point. The angle of the tip won’t be limited by the natural crystal shape; instead the spear tip can be made both strong and sharp.

Obsidian is generally made out of high silica, rhyolitic lava. These high silica lavas are very viscous and therefore crystals don’t grow rapidly on them, making obsidian formation easy. Different obsidian compositions and structures do behave differently during processing, so some obsidian sources were highly prized and rocks that match in chemistry were traded across thousands of kilometers, covering entire continents.

-JBB

Image credit: John Atherton (Creative Commons):https://www.flickr.com/photos/gbaku/1287124990/