I just had to Google that one.
I had thoughts of regular old rocks with bullet splashes on them - the only thing some "hunters" ever shoot.
But, no.
They are apparently real rocks, allegedly mined from someplace in the Rocky Mountains that contain so many irrestistible trace elements that deer crowd to them like free appetizers at the local bar. Here's a
LINK.
If it's true, where are the herds of deer milling around and licking rocks here in the middle of the Rockies? I haven't seen that just yet!