According to Robert Barefoot,
in his book, "Barefoot On Coral Calcium..." (p. 24) Robert
Barefoot states:
"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
land mass, 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."