Obsidian

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when lava cools rapidly with minimal crystal growth. It is commonly found within the margins of lava flows, where the chemical composition (high silica content) causes a high viscosity which, upon rapid cooling, results in a natural glass forming from the lava. Obsidian is hard, brittle, and amorphous; it therefore fractures with very sharp edges. In the past, it was used to manufacture cutting and piercing tools.
In Peru, you can find black obsidian and the so-called mahogany obsidian(black mixed with brownish red)