Gibber, pebble and cobble are alternate names of stones. Rocks are different. According to the Oxford English Dictionary, a stone is “A piece of rock or hard mineral substance of a small or moderate size” whereas a rock is “A large rugged mass of hard mineral material or stone.” There! Clear as Mud!
The cache is hidden in a pile of rocks - is it a Cairn? - or is it just a heap of stones? You decide.