Why are the overwhelming majority of skeletal systems calcium based instead of some other mineral? Is there any record of organisms with different mineral based exoskeletons? Edit : thanks for the replies everyone unfortunately there wasn't a definitive answer but the main points brought up were abundance of calcium ions, it's ability to easily be converted to soluble and insoluble forms and there was one person who proposed that calcium is used for bones since it is a mineral that's needed for other functions in the body. I look forward to read other replies.