"This is a stone."
Камень is nominative. Камня is genitive. We need to use the nominative in "This is a stone".
Because you are saying "this is a stone", and "this" is neutral. Only if you use ето as a definite article it would be ето/ета/етот. Это камень= this is a stone. Эта камень= this stone. In english it is quite hard to understand the difference, german learners have the same issue with Das vs. Diese/Dieses/Dieser Das.