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, and when picking up a rock it will be added to the amount of carried rocks, instead of appearing in the inventory. This means it's technically possible (although more convoluted than just using the normal method) to pick up five rocks above water, and then not have to pick up any rocks after going diving, before entering the crab pen.
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