I use Metagenics' calcium hydroxyapatite supplement (Formula Cal-Phos -- Cal
Apatite).  It is made from the bone marrow of sheep.  If you have a
"hunter-gatherer metabolism", then you probably aren't getting an optimal
amount of calcium, unless you're eating bones and bone marrow as your
ancestors did.  Just for the record, I don't take a calcium supplement
because I'm worried about osteoporosis, but rather because I've been told
that it should help to balance my metabolism.

Cheers,
-- Aaron Wieland