Nice little button collection , to my eyes , CH !
The 'maltese cross' buttons pretty much appear to be from either marble or alabaster .... perhaps the round-slab ones are marble as well ... might have been used as the stiffbacking for a cloth-covered button (needing no slots/holes thru slab for attachment) .
I'd guess , from assuming that the story of their origin is 100% true , that the 'crosses' may have been string or thread cut (using sand/dirt/ash as the abrasive) ... (you can easily see for yourself how well a simple string can cut , if you try one out on a scrap of PVC or ABS pipe) .... took alotta work to cut them , I'd wager !
IMO , the chances that these items were fashioned in a concentration camp would be in keeping with their appearance .
Thanks for posting them !