Great question. I'm not geologically inclined in the slightest and have no idea. Two different types... The first is jagged, gray, pre-river river rock (i.e. rolled off the mountains and down to the Poudre, without going in to get smoothed out). It looks good in a jumble, and when I kept loaches they loved the hell out of it. The other type I got free from a local landscape supply yard (the guy laughed out loud and told me to just take it when I told him how much I wanted) - more rounded, dark gray, with some white and red areas.
I'll get some pics posted when I have a chance...
And here you go.