This is absolutely ridiculous; Calcium carbonate causes kidney stones.
Coral calcium only 23% is absorbed, compared to 75% of the type of calcium that I take and that my company uses in our product.
The reason we use algaecal in our supplement "Cell Synergy" is that it is the most absorbed form of calcium: http://www.essentialsfactor.com
It also contains a "good" assortment and proportion of other 80+ trace minerals. We mix it with other important minerals that many are deficient in, along with immune supporters, liver detoxifier, etc. If you go on site to see calcium comparison chart and the reference.
Not just to give such a blatant advertisement, i'll tell you that you can buy it online just by itself with cal/mag its called AlgaeCal. But you really do need these other minerals in our product too. Your free to inquire with me about why the other minerals together with the calcium is really great. See http://www.essentialsfactor.com.