Yes. The gender of the following noun will tell you if it's это or эта, although we, as learners of Russian, find it very difficult to determine what gender a word is in Russian.
How do i know the difference between "This is a stone" and "This is stone"?
I am getting that the correct answer is "it is stone." Can anybody help me out here? That's different from "this is 'a' stone."
In this case both are valid, because "камень" can refer to both an object and the material.
When is это used as an article and when is it used to point something out?
I believe это is always used when presenting/pointing out something. It gets confusing because это is also the demonstrative adjective for singular neuter nouns in the nominative and accusative case.