Notes:

To the few that may not know,
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.
It is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerization degree of the lava. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Obsidian is hard and brittle; it therefore fractures with very sharp edges, which were used in the past in cutting and piercing tools, and it has been used experimentally as surgical scalpel blades. (From the Wikipedia page on Obsidian)
→ Many different civilizations in Mesoamerica used a weapon known as the Macuahuitl, a wooden sword lined with shards of obsidian on the edges. The legend of the Macuahuitl is one coated in blood, the obsidian used was so sharp that it was commonly stated that a good Macuahuitl could take the head off of a full grown horse.