If you read the original paper (https://www.frontiersin.org/journals/psychology/articles/10....) then they go into more detail on the piles of pebbles and what got taken; the graphs in figure 4 (https://www.frontiersin.org/journals/psychology/articles/10....) make it very obvious that the chimps loved the crystals.

(an "euhedral" crystal is one with lots of obvious facets, an "anhedral" one is one that's been rounded down into a more pebble shape.)