Large Obsidian Shard

Large Obsidian Shards are pieces of dark volcanic rock that are strong and can be used by Blacksmiths to craft strong armors with anti-magical powers.

Source
This item can be mined from Small Obsidian Chunks and Large Obsidian Chunks which will sometimes appear after defeating certain mobs inside Ahn'Qiraj such as, the Anubisath Defender, the Obsidian Eradicator, and Moam. A Mining skill of 305 is required to mine the chunks.

Blacksmithing

 * Black Grasp of the Destroyer
 * Jagged Obsidian Shield
 * Obsidian Mail Tunic
 * Thick Obsidian Breastplate