Material Recommendation: rocks/gravel in galleries?

Hey everyone,

We are receiving an exhibition with two horse mannequins. The mannequin bases are made out of metal tubing and need to be covered while on display. We have discussed using rocks or gravel to cover the mounts to make it look like the horses are walking on terrain. Any suggestions on products that would be safe to use in a museum (clean, pest-free)? We ordered a sample of pre-washed aquarium pebbles but they still seem a little dusty.

Thanks in advance!