[tom_mellior]

> Many of the beaches are on islands which are essentially coral heads above the water.

Afaik most Caribbean islands are of volcanic origin. Barbados is a notable exception, it's a coral island.

[Accujack]

Actually, the caribbean is the area I'm most familiar with. Lots of small islands in the Bahamas chain with sand and coral beaches.

Perhaps islands in deeper water are volcanic, but I still doubt that mining that much olivine anywhere near the place it would be used is possible.