callous / callus
Hardened in feeling versus a thickened patch of skin.
Callous is the adjective: a callous remark. A callus is the noun for thick skin. The verb is usually callus: her hands callused. Do not write a callous on the heel.
Examples
Use That was a callous reply.
Use He had a callus on his palm.
Avoid He had a callous on his palm.