Bone Charms are made from whale bones, and provide Corvo with extra benefits when equipped. Corvo can find Bone Charms by using The Heart item he acquired from The Outsider. When equipped, The Heart will pinpoint where and how far away the Bone Charm is located.
An important aspect regarding the Bone Charms in Dishonored is that they only appear after the first mission. Each playthrough only spawns a certain number of Bone Charms, although there is a lot more available in the game. Corvo starts with three slots to equip charms in, but can purchase more at the Hound Pits. The Bone Charms are randomly assigned when starting a mission, so if you do a mission 3 times and get the same Bone Charm, it will always be different.
