Fayrg bones are associated with the Prayer skill. They are obtained from ogre coffins, which will often break lockpicks and decrease players' thieving abilities. Fayrg bones give 84 Prayer experience when buried. They are known to come up as daily challenges.
This table can be used to compare and decide on how to train with the bones for the most desired cost versus experience gained. Notice how the cost in coins per XP point varies greatly depending on how the bones are used — but also remember that the different methods have different training speeds.