I don't know where it generally comes from, but I remember seeing places places in Florida far inland where all of the ground was just old crushed coral. It wouldn't harm the ocean much to quarry that material.
Somebody should manufacture and sell a consistent, well-composted shredded bark product as an alternative to peat. When I have used it before composted bark works as well as or better than peat in potting mixes. It's surprising that there are hardly any commercial products like that on the market.