What's the difference between a rock and a stone? Can we say ( while teaching the opposites to the children) rocks are hard but cotton is soft. Someone told me that my sentence is wrong. That should write stones are.... instead.
Not sure if it helps, but to me, a stone is small, a rock is large, and stone is the material.
The boy liked collecting stones in his garden.
The men were climbing the rock.
My house is made of stone.
So with reference to your question for opposites, I don't think it matters.