No one has responded yet so I guess I'll take a stab at it. Keep in mind though that I am a rock nerd, not a healing crystal nerd.
I haven't heard much about using rocks in crystal healing, but since rocks are composed of mineral crystals, albeit much smaller grains than the usual big crystal specimens, its logical to wonder if they have some share in specific metaphysical properties attributed to the larger mineral crystals. Very often they have just one or several major minerals and one or more trace minerals or metals, which, again, may or may not affect the metaphysical properties. It can't hurt to experiment. At the very least you'll have some nice specimens to look at next to the crystals!
In general, limestone and marble mainly contain calcite, granite generally has an abundance of quartz and feldspar, and 'lava rock' is dependent upon how it formed and is often made of whatever melted rocks and material were in the vicinity (so if you're hoping for special lava rock properties, it may depend on where and what it came from).
The limestone and pumice/obsidian (if that's what you mean by lava rock) are probably engraveable by dremel, but the granite/marble may be tougher. A diamond grit tip or more heavy duty tool may be needed. If you try it on the harder stuff, keep the rock and tool grinding edge wet as you carve to cut down on dust and possible hot-'nuff-to-melt-the-tool heat. (And put the rocks in a heavy clamp so they don't fly into stuff!)
"Generic" may be a way of saying the origin and specific mineral composition of the rock is unlisted (i.e. it's not some granite from one specific and well-known quarry sought out for its especially hard nature created by some rare trace minerals found only in the region).
