Stone of Agony
It is believed by some that the Stone of Agony is actually a companion relic of two other items: the Mask of Truth and the Lens or "Eye" of Truth (see Lens of Truth). And so when all three items are used collectively a traveler can often find his way much easier. The stone material itself was cut long ago from an enchanted Gossip Stone of Hyrule which is where it aquires its sensitivity to secrets and clues.
The Stone of Agony reacts to nearby secrets by (if you have inserted a rumble pak) rumbling if you go near something. The Stone of Agony is shaped like a Nintendo 64 Rumble Pak. (See also: secret grotto)