Obsidian: The Black Heart of Aran'sha

"A stone knife? Are you kidding me? That's the best you have?" The stranger snorted, but the weapons trader behind the counter just smiled.
"My friend, I would never disappoint a valued customer. It's not just a "stone knife", it's a Kaharan obsidian knife and worth a years hard labor. Let me show you."
The adventurer's brown eyes widened as the weapons trader drew the knife from its ornate sheath. It was a work of art, with swirling patterns on the void black edge.
"Kaharan obsidian is the sharpest stone in the world," the trader explained. "It can cut through anything." He reached out and sliced through a piece of rope with a single stroke of the knife. The rope fell to the ground in two perfect halves.
Respectfully the wanderer took the knife from the trader and examined it closely. It was perfectly balanced, and the blade was so sharp he could feel it cutting through the air. He had never felt anything like it before.
"How much?" he asked.
The trader smiled. "A year's hard labor, as I said."
The stranger hesitated. It was a lot of money, but he knew that this masterwork was worth it. He nodded. "Done."

  Obsidian, the black heart of Aran'sha, can only be found in the Obsidian Wastes to the west of the continent. It is mined in perilous conditions by a small army of workers. The razor-sharp edges of the obsidian, suffocating heat, and frequent volcanic eruptions in the area make the miner's life a constant hell.
  The shards are brittle in their original state, but still incredibly sharp, making it a difficult and dangerous material to work with. Caravans under heavy guard carry the precious mineral to the cities, where only the stonesmiths of Ri'kahar know the trade secret to hardening the material, tempering it to a gleaming edge that can cut almost anything.
  Only a few weapons are made each year, and as commodities they know no equal on the continent. A single obsidian dagger is a prized possession, and a sword is worth a king's ransom.