Actually, just on the topic of rockdorkery, I'd point out that even when you're trying to separate out stones by what they look like, you're still separating stones by what classification they are.
Igneous stones look different based upon how they cooled. Magma is like water with a lot of silt or other things that don't dissolve in a mixture - if you freeze it just after you've shaken it up, everything looks uniform, and the fastest cooling stone is obsidian, which looks pure, jet black, and has a smooth appearance. If you let the whole thing freeze slowly enough that all the grains settle, you get granite, the slowest-cooling felsic intrusive stone, which has those clearly stratified brown and red and black and white flecks and patches in it.
Intrusive igneous stone is different from extrusive igneous because extrusive stone cools faster, so it has less time to separate. This makes extrusive and intrusive igneous formations look significantly different, as intrusive stones have patterns in them, while extrusive stones at most have little swirls of color.
It's a lesser concern, but mafic magma is magma with a high iron and magnesium content, while felsic magma has let its iron and magnesium separate and solidify while the rest of the magma (with a lower freezing/melting point than those two metals) was still liquid-hot. This also makes felsic stone lighter-colored, because magnesium makes very dark stones.
Obisidian => Basalt (Mafic Extrusive) =>
Rhyolite (Felsic Extrusive) =>
Gabbro (Mafic Intrusive) => Granite (Felsic Intrusive)
Smooth ================> Medium =======> Grainy
I'd try making some of these myself, but I'm not really sure how to go about making the sort of watercolor effects that would make the things I make blend in very well... I'd just try to do my standard pixel art work with shading on a rough splotch of what the stone looks like.
Of course, there's also the fact that I was doing job icons and sort of drifted away from that
(I'll get back to it when I'm done writing up some changes in a suggestion thread I've been back-burnering, really!)