In his latest book, "Barefoot On Coral Calcium..." (pg. 24) Robert Barefoot writes -

"It is important to recognize that two broad, but distinct types of coral calcium are used as health giving supplements by the Japanese and many people throughout the world. The first type is fossilized calcium that has been deposited on the landmass, or washed up on to beaches. The second type is taken directly from the seabed. It is marine bed coral. This seabed coral is the coral that has dropped from the reef or is processed by reef inhabitants. This type of "coral sand" has been washed to the ocean floor by wave actions. Marine coral is closer in composition to the living forms of corals, because many minerals and organic elements are retained, in comparison to fossilized, land-based coral.
There are important differences in composition between fossilized (land-based) coral and marine (sea-bed) coral. Marine coral contains more magnesium, and the balance of calcium (24%) to magnesium (12%) content of this second type of marine coral is close to 2:1. This 2 to 1 ratio is the ideal ratio for calcium and magnesium intake in the human diet. My research has led me to believe strongly that the natural, magnesium enriched, marine coral is to be strongly preferred as a health giving supplement over land based (fossilized coral), which contains less than 1% magnesium. This superiority is due to its retained, ideal, ionic balance of calcium and magnesium in a 2:1 ratio, and the fact that a host of other nutrients were also washed out of fossilized coral during weathering processes. These issues are considered in greater detail in Chapter 4.