A rock can be obtained in the Mogre Camp by picking them up. They are required to enter the Crab Pen which is primarily used during Recipe for Disaster. It is never seen in the inventory as they disappear upon entry.
A glitch exists where it is possible to see it in the inventory by using Telekinetic Grab to obtain the rock from the Crab pen. The animation of picking them up on land is the same as picking them up underwater.
Subquest of Recipe for Disaster