Georgiaman,
Might I suggest checking out a few websites with photos of rock you'll get a better idea of what you're shopping for than what my lame descriptions can do. Fiji tends to be a little dense, with lots of purple corraline, porites coral, and sponges. Carribian tends to have gorgonians, macroalgae, and more pink/red corraline. Density can vary due to farming (type of rock) it's mostly aragonite.