I used Tahitian Moon Sand from Caribsea in my 55 and I really like it. It's fairly cheap, jet black, and a nice mix of sizes. It won't compact like true sand, but it's not as clunky looking as basic gravel. Google shopping link
Here's a picture of it submerged. The color came out a bit yellow, but you get the idea. Looks 'natural,' though I've never seen anything like it in nature
The brown flecks are MTS snails, which are sold separately.