I think you're referring to Gamer Escape's interview with Yoshida:

Gamer Escape: When I’ve converted some strength rings I’ve been getting Mind Materia! Is the Materia you get from an item random? Is there a chance of getting better Materia if you repair the item before converting it?

Yoshida: First of all, it is not random. There is a table we have with each item in it and that item will have a possibility of turning into one of these. That table is currently a secret, but we’re going to be releasing it very soon so players will know what items will turn into what Materia.