There has been a recent surge of promotion selling "coral calcium pills". Selling ground up coral skeleton as a natural source of calcium is an endlessly recurring scam that hucksters pick up on every few years like clockwork. My colleague Wolf Hilbertz has a bottle of coral tablets someone was peddling in the 1960s.
There are many natural sources of dietary calcium that don't require killing corals. It is important to keep a balance between your calcium and magnesium intake. One recent coral calcium ad I got said that people in Okinawa are healthy because they get loads of calcium in their drinking water, but of course this is true anyplace in the world where the water is hard because it comes from aquifers in limestone rocks. The negative side is that they can have very high rates of kidney and bladder stones, as well as gastric cancer because limestone groundwaters are loaded with nitrate. Oyster shell calcium should be fine, and the oyster wasn't killed for the shell (somebody else ate the meat and chucked the shell), but organic forms of calcium are probably more readily absorbed.
Coral skeletons are very pure calcium carbonate, unless the coral is from a very muddy place and has included sediment grains, whose composition will vary from place to place. It contains no meaningful nutrients except a lot of calcium with traces of magnesium and strontium, but it is fundamentally no different nutritionally than any other form of limestone, including oyster shells, or ground up limestone rocks or marbles, which are certain to be nutritionally richer in trace metals. Actually the best form to take calcium is in organically chelated form, say calcium gluconate, which is much more readily absorbed by the stomach. The claim that corals build strong bones in humans than other forms of calcium is pure hype!
Corals are vanishing everywhere and any mining of live corals for this trade should be stamped out. Mining of dead corals should not be a problem unless they are part of a dead reef that is still helping to protect the shore from erosion. There are such vast supplies of limestone on land, that these shouldn't have to be mined either.
A recent web site presents information on coral mining in Okinawa, see http://www.cureamerica.net/, "coral wars", claiming that there is large scale mining of both reef coral sediments, and limestone formations on land, which are referred to as "dirty" limestone. It further claims that they are mining coral sand without harming corals and that the reefs are in excellent condition. While it is true that limited amounts of sand mining can be carried out without harming reefs, in most cases the mining operations increase suspended sediment turbidity, and plumes of this turbid water drift onto corals, where they severely stress or kill them. My late friend Zenji Yoshimine, lost his life photographing the long-term decline of Okinawa coral reefs. Most were killed by mud washing from land after unwise large scale deforestation and land bulldozing operations. Some died from sewage pollution. And in 1998, almost all the surviving corals died from high temperatures caused by global warming. Sadly, only a very small part of the once stunningly beautiful Okinawa coral reefs now remain in good condition, and the few still remaining are threatened by dredging and land clearance to make a new US air force base.
The report claims that the coral being mined has an optimal ratio of two parts of calcium to one part of magnesium. I published the first measurements of seasonal variations of magnesium and strontium in coral skeletons, and assert that no coral skeleton has this ratio. Only around 1% of the coral skeleton is magnesium. Higher "optimal" ratios are common in old limestone formations on land. Although corals themselves have very little magnesium, other marine organisms, especially sea urchins and coralline red algae make different mineral forms of limestone that may have up to 10 or 20 % magnesium. If it is being called "coral" limestone this must be an inaccurate claim for publicity purposes. Any limestone with a 2:1 ratio of calcium to Magnesium is not coral skeleton; it is limestone from other organisms, or ancient rock formations that contain the mineral dolomite, a limestone with a 1:1 Magnesium to Calcium ratio mixed with calcium carbonate.