Two broad techniques have been described. A heavy axe-like blow with a pointed stone is used on oysters cemented to rock, while a flatter pounding stone is used to smash unattached snails and crab shells against a rock surface. Individuals reuse favoured stones and leave them at productive spots.
The behaviour is coastal and local. It has been recorded chiefly in Laem Son National Park in Thailand and on nearby islands, and neighbouring inland groups do not do it, which is the same pattern of geographic patchiness seen in chimpanzee and capuchin stone use. Archaeological work at these sites has recovered used stones in older shell deposits.
There is a conservation twist. This macaque was long regarded as a weed species thriving alongside people, but the wild population has fallen enough that the species was reclassified as Endangered in 2022, driven largely by trapping for biomedical supply and by habitat loss.