I used a big serrated knife on my sample of cinder, and could see slight, light marks. But my DH didn't notice them when I asked him to look at the sample carefully. Then I took a hammer to my my truffle sample, created a mark smaller than when I banged it with the heavy blunt handle of a butter knife.
I also poured boiling water, and set a pot of boiled water, on the samples, followed by ice. And scrubbed them with an SOS pad, and magic eraser. No changes.
150 grit sandpaper caused the samples to lighten where I rubbed them, but rubbing a little canola oil on them made all the marks hard to find. So I scrubbed again with 240 grit, and saw little change.
After all that, DH sees "nothing" wrong with the surface of the cinder sample. I see that I've made it less opaque, that whatever top finish they use has been worn off and tiny white specks now show through (and my knife marks are gone) - but it looks okay to me, more like a natural material, actually. The sides of the samples are lighter when dry than the top, the way honed granite is lighter than polished. The bottoms of the samples have a thick, plastic-y finish. I tried to use 240 grit sandpaper followed by magic eraser on one side to get it "finished" like the top, but I didn't make much progress. If I make it out to Lowes today I might buy finer grit, just for fun.