Stone of Agony

The Stone of Agony is an item featured in Ocarina of Time. It is a magical stone that will vibrate when an unseen secret is close to its holder. It has a unique shape, like that of its real-life counterpart, the Nintendo 64 Rumble Pak, and also possesses the symbol of the Triforce etched upon its surface.

Location and Uses
Link acquires the stone as a reward from one of the cursed residents of the House of Skulltula in Kakariko Village after destroying 20 Gold Skulltulas. Possessing the stone within the game will activate the Nintendo 64 Rumble Pak and cause the player's controller to vibrate when close to a hidden secret. Majora's Mask made this a default feature of gameplay, so that the controller would shake whenever Link came close to a hidden secret.

The only secrets that the Stone of Agony will detect in Ocarina of Time are hidden holes in the ground. Majora's Mask adds to this by also detecting hidden treasure chests (with the exception of invisible chests that can be seen with the Lens of Truth). Interestingly, in Ocarina of Time: Master Quest, the Stone of Agony will also cause the joypad to rumble for hidden chests, at a lower frequency than in Majora's Mask, but only in the Spirit Temple. Since Master Quest was released after Majora's Mask, this feature was added, probably due to the increased difficulty of the Spirit Temple.

The stone is a companion relic of two other items: the Lens of Truth and the Mask of Truth. When all three items are used collectively, a traveler can often find his way much easier and have a firmer grasp of the true nature of their surroundings. The stone material itself was cut long ago from an enchanted Gossip Stone of Hyrule, which is where it acquires its sensitivity to nearby secrets and clues.

The Stone of Agony is present in the Wii Virtual Console release of Ocarina of Time, but since the Wii Classic Controller does not support the functionality of the Rumble Pak, the item serves no purpose whatsoever.

Shard of Agony
Re-engineered in Ocarina of Time 3D as a new item, the Shard of Agony, the Stone of Agony's legacy remains solid; though the Shard and the Stone serve virtually the same purpose, the Shard of Agony will release an audible "pinging" whenever a secret is nearby, accompanied by a flashing icon of the item itself in the top-left corner of the screen. The rumble feature familiar to the original Stone of Agony was removed in favor of these visual and audible alerts.

Theory
The Stone/Shard of Agony is speculated to be part of a set of three objects, including the Lens of Truth and the Mask of Truth, that were created by the Sheikah tribe to give their holder untold power in terms of perception of the truth of their surroundings.