The use of natural coral as a bone graft substitute is common in Europe. However, the bone-coral bonding mechanism remains elusive. A rat subcutaneous model was used to demonstrate changes at the surface of resorbable calcium carbonate in the form of natural coral. Histological results indicated in vivo formation of a calcium phosphate (CaP)-rich layer on the surface of the coral confirmed by backscattered electron imaging and X-ray microanalysis. There appears to be a combination solution-mediated dissolution/cell-mediated degradation of the natural coral with subsequent surface conversion or precipitation. The end result is a CaP-rich layer on the coral. Though this layer has been observed previously, it was originally thought to be a histological artifact. This result is similar, however, to what is seen with Bioglass and glass ceramics and may also explain the good bonding of bone to hydroxyapatite. The fact that this layer is also present on natural coral after implantation in soft tissue sites may explain the intimate bone apposition observed when natural coral is placed in bony sites.