Koi is using stones to chew her food

One of my Koi is clearly using a mouthful of rough gravel to chew her
food when I put a piece of meat in front of her. Then she spits the
gravel out. This is in a 60 gallon aquarium. The bottom is covered
with irregular rubble granite gravel with approximately 1/4"
dimensions. The fish takes the food near the surface and then goes to
the bottom and pulls up several such stones clearly chewing her food.
Some of the small pieces of food will escape from her gills during this
and then she spits out the stones. The other fish (I have only two in
there) does not do this and the "tool user" does not pull stones into
her mouth in this way (and keep them there) when she is foraging the
bottom. Of course, they both move stones around with their mouths when
they are searching. She also does not do this when she is eating
"flake" food.