It's a little like tartans--no real rules, just good taste.

Pick what looks good, and a black stone would look very good with a MacGregor tartan.

I have an amethyst for use with my Robertson Red great kilt, and it looks good, too. The two tartans are close enough that I could recommend that as well. In fact, if you can't find one and want one, PM and and let's talk. I think I have an extra I bought years ago with some other stuff that I could part with at a very reasonable price.