Death by pet rock

Gwen Jackson's death in 1982 seems to be the only instance of a 'pet rock' being used as a murder weapon.

Although I think the media sensationalized the case. The rock in question seems to have been a rock given as a gift. It wasn't an actual 'pet rock'.

Tucson Citizen - Mar 19, 1982

However, the manual that came with Pet Rocks, "The Care and Training of Your Pet Rock," did include a section on "Attack Training" your pet rock. So their use as a weapon was anticipated.