What black volcanic glass is known for breaking with very sharp edges? Obsidian is the answer.
Obsidian forms when silica-rich lava cools so quickly that its atoms do not have enough time to arrange themselves into crystals. The result is a natural volcanic glass rather than a conventional crystalline mineral. It commonly develops along the edges of rhyolitic lava flows, volcanic domes, or places where lava meets air or water.
Its glassy structure makes obsidian hard, brittle, and capable of conchoidal fracture—the smooth, curved break also seen in manufactured glass. Those fractures can produce edges sharper than many metal blades, which made obsidian valuable for prehistoric knives, arrowheads, spear points, and cutting tools. Experimental surgical scalpels have also used obsidian edges.
Obsidian is usually black, but it can also be brown, green, red, or other colors depending on its chemistry and microscopic inclusions. It is often confused with pumice or scoria, but those rocks are full of bubbles and have a porous texture; obsidian is dense, smooth, and glassy.