Mostly rock, or wood. That's not just pumice or barbecue coke. Any rock can undergo the volcanic process - then get worn down or crushed into regular rock again. I'm looking for red.
Volcanic rock can be gray, or red, or black.
Or a mixture. It's common to find natural clay tessera embedded in the rock, usually pink or gray. The rock can be matte and full of airholes or dense and shiny, the grains fusing together like tiny jet beads. It allows work on a small scale, and doesn't appear to be carved. And it's relatively light - an important factor in fountains meant to be portable.