A fair part of the coarse sand, fine gravel and stones that I can collect out of the local river is of volcanic origin. This stuff is not all created equally. I'm fond of the rounded pebbles and stones of scoria and lava. These are either black or iron-red and full of bubbles, so they're fairly attractive and much lighter than most stones. Java fern anchors to it very easily.
Some volcanic material is mostly glass.
Obsidian and pumice can be quite unique and attractive. But natural volcanic glass is impure and may not be as stable as man made glass. My tap water is quite high in silica because of reactions between ground water and volcanic material in the aquifer. Sands and gravel composed of volcanic material might contain some of both of those types. Probably the largest component is just fine-grained, somewhat weathered mixed bag of different kinds of tuffs and lavas.