it does seem that concrete, if mixed a little on the liquidy side, will make a reasonably-detailed reproduction of nature objects pressed into a 2-part silicone mold. these have cured for a couple of weeks and they are quite sturdy...only the thinnest of edges snap when pressure is applied.
on the other hand, after curing for about a week i subjected the concrete pieces i made with black drawing ink to a little washing with soap to see if i could get rid of the ink-coming-off-on-my-damp-hands problem.
the 2 that were made entirely of ink and concrete powder just simply started to disintegrate at the edges. and then eventually crumble. that black color with the aggregate showing is REALLY nice though...glad i have some black pigment powder on it's way to me as i type!
the good news is that after the wash-up, this little guy (who only had 1 drop of ink added to the water at concrete mixing time) made a nice color concrete, the embedded jumpring is holding, it didn't disintegrate, and it's not leaking ink onto damp hands.