What material is commonly known for its brittleness?

Obsidian is a naturally occurring volcanic glass formed from rapidly cooled lava. It is known for its glass-like appearance and sharp edges, which make it brittle. Brittleness refers to a material's tendency to break or shatter under stress rather than deform. Obsidian exhibits this characteristic because it lacks a crystalline structure, making it more prone to fracture when subjected to forces.

In contrast, the other materials listed have different physical properties. Perlite is a lightweight volcanic glass that is often expanded for use in horticulture and insulation. While it can break or crumble, it is not as brittle as obsidian. Brick is generally durable and designed to withstand compression, making it less prone to brittleness. Wood, being fibrous, has some flexibility and can bend rather than break under stress, so it does not exhibit brittleness in the same way as obsidian. Hence, obsidian is distinctly recognized for its brittleness compared to the other options.