Stones in Coffee

This evening, while roasting up a batch of Mexican coffee from the Comalapa region for the coffee drinkers at my office, I noticed something strange spinning in drum with the beans. After looking closer it was obviously a stone of some type. Stones are really not fun to get in coffee, primarily because they can wreck a coffee grinder by jamming in the burrs. Since many coffee grinders are rather expensive, this can be a fairly serious problem. Unfortunately, stones seem to be somewhat common issue for coffee roasters, and larger operations actually use machines called destoners to remove stones from roasted coffee.
Stone in with the beans… I actually found it down underneath the coffee, but this is just to show the size.

Once I dropped the roast and got the beans cooled down, I sifted around and found the stone pretty quickly. Turns out it was a small piece of concrete. I haven’t ever run into this with beans I’ve gotten from La Bodega before, so I’ll keep an eye on it. I’d sure hate to ruin my (or anyone else’s) grinder.
Close up of the offending stone