Hellforge

From Diablo Wiki

The Hellforge is a magical forge found in the Burning Hells which has the power to destroy a Soulstone. The Hellforge is protected by an Overlord called Hephasto the Armorer, who holds the Hellforge Hammer required to smash the Soulstone. During Diablo II, your hero completes a quest named Mephisto's Soulstone, where he/she smashes Mephisto's Soulstone using the Hellforge and the Hammer.

However, you do not need to have Mephisto's Soulstone in your personal inventory. Either this may be a bug, or somehow, Tyrael's magic allows him to teleport the soulstone directly to the Hellforge when you are there and ready to use it. All you really need is to wield the Hellforge Hammer, and whether it's on your inventory or in your stash, you will still be able to complete the Soulstone-smashing quest with it. Nevertheless, the soulstone is found to have vanished once you reopen your Stash.

When the soulstone is destroyed it gives several perfect and flawless gems (in normal dififficulty) and mid/high runes (in Nightmare and Hell only). It is common to offer the reward for characters hellforge quest in exchange for a rush.