Raurg bones are associated with the Prayer skill, giving 96 prayer experience when buried.
They are obtained from Zogre coffins, which will often break lockpicks and decrease players' thieving abilities upon opening using thieving methods. They can also be obtained by looting a zombie impling jar. (Zombie impling require level 87 hunter to catch.) These bones are impractical to train prayer with, as they are significantly more expensive than Frost dragon bones, therefore it is recommended to sell raurg, fayrg, and ourg bones accumulated from ogre coffins.
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.