Join Date: May 2007
Location: Webster Groves, MO
It looks like some sort of igneous rock (granite, basalt, etc.). Your best bet would be to go to a local landscape supply place and see what they have that you like.
I just took a walk in our construction yard and found several that would work ranging from purple to black to grayish-green. Just be sure to stay away from limestone as it will raise your pH.