You've never seen the infomercials?
Yes... it's actually made of coral. The kind I take is made from coral sand, which is harvested from the bottom of the ocean floor off the coast of Okinawa Japan. They don't bust up the coral, they take the sand that's on the bottom of the ocean. Some places take it off the beach... but supposedly the best stuff comes right out of the ocean because it hasn't been weathered.
There are tons of other kinds of calcium supliments, but supposedly calcium from coral is the most easily absorbable. They're saying if you take a regular calcium suppliment made with calcium carbonate, you absorb about 10% of that calcium... milk, you get 20%... but this stuff your body absorbs 100% of it in 10 minutes.
I'm not sure of all the facts, but it makes damn good sense to me after reading a few books. And my knee completely healing is enough proof for me.