I am told that the word hart was in the past a substantive (not only an adjective) with the meaning of "glade", like a "forest grade". As an example: Steinhart had the meaning of "the glade of the stones". Can you confirm or deny this?
Indeed there used to be a noun "Hart", "Hard", "Hardt" or "Harz" used in the meaning of a forest, or a mountain grown with trees.
Today there are still remnants of this meaning in names of locations, regions or towns. The most known region being "Der Harz", a mountain range in middle Germany.
This may also be the case for the then names of the village and the ruined castle "Steinhart", Bavaria, but the common meaning of the composite adjective "steinhart" is (and presumably always was) hard as stone, which is not related to a forest.