Blood clotting is initiated by the activation of several clotting factors, which are in turn activated by calcium. Thus, calcium is the mineral ion that plays the main role in blood clotting. In the absence of sufficient calcium, the clotting process may be impaired, leading to excessive bleeding. It is important to note that excessive calcium can also lead to abnormal clotting, so maintaining a proper balance of calcium in the blood is crucial for proper clotting function.