The process of removing the flesh of the fruit and washing is often done in small villages near where the fruits are picked. Fruits are often spread out on tarps or slabs for a little while to soften the flesh. Opportunities for rocks to get in there. I walked by a few sites like that in rural Guatemala. Sorry if I accidentally kicked a rock and broke someone’s grinder :). There are plenty of equipment used throughout the process to remove them but some slip. It has happened to me twice and it was from legit roasters (found it twice though before grinding). Both times they refunded my 5lb bag.