Yes, yes, I know, obsidian is volcanic. But this obsidian is also sedimentary. The road gravel here is full of rounded clasts of this volcanic glass. I don't know where it crops out, or if the original erosion surface even exists anymore, but clearly, there was at one point an obsidian dome or flow that was exposed to enough running water that cobbles and gravel were transported some distance to round them- which wouldn't necessarily be very far.
It's fairly easy to get some very, very pretty examples of conchoidal fracture from these. The "trick" with glassy rocks like this is to *gently* tap the rock. If you whack on these as you would with a more typical stone, you'll end up with shattered splinters, and more likely than not, blood. The other part of the trick is to "pluck" rather than strike. It's probably best to show this in a crude picture:
Striking the obsidian cobble, again, *gently,* as shown will cause it to fracture approximately as illustrated by the thin dotted line. I've spent many hours "breaking glass" to make arrowheads, and have no hesitation doing this with my bare hands (as Dana can attest), but do be careful. The edges are pretty much as sharp as it's possible for any material to be- much sharper than a razor blade. I don't do this with kids around- I don't want them to see how easy it is before they've developed a healthy respect for the stuff and its edges. Another trick is to immediately take a decently large (fist-sized), fairly soft rock of another type, and "brush down" the edges to dull them. Fun! Remember, every time you break open a rock, you're seeing something no one in history has ever seen before.