Stonecore - Vitreous Stone Drake

The Stonecore is a 5-man dungeon located in the top of the Temple of Earth in Deepholm. Solo access requires a flying mount and is located on the west-northwest side of the temple.

Slabhide, the second boss, drops the Reins of the Vitreous Stone Drake.

Once you defeat Slabhide, a blue glowy portal will appear directly behind him that will port you back to the dungeon entrance.

***As of Patch 6.2.3 a new rare mount, Reins of the Infinite Timereaver, has been added as a random drop from any Timewalking Dungeon Boss, which includes the bosses of Stonecore during any Cataclysm Timewalking event. (requires level 86+)

For more information on the new Timewalking Events, see this great this great guide at Wowhead.