I'm working in a videogame set at the end of the Third Age in Middle-earth, and I would like to properly use Middle-earth materials, flora and fauna. I've been looking at MERP (a tabletop RPG) publications, they're great, but I'm aware of them adding non-canon elements. For example, adarcer metal.
The problem is, how do I know what is non-canon? Is there any compilation with canon material? Some publication or website. Based on this question, I suspect there is none, but checking Tolkien's novels and letters by myself would be pretty crazy.
Ideally I would have descriptions and explanations of their special properties. If not, I'll use MERP stuff, but at least it would be great to know what's canon.