If you check the dictionary, it gives "equity" as a noun and not an adjective. That said, I don't think most people would object to its use as an adjective in the phrase "equity investments."
I remember a footnote in a materials science textbook I had in college. It pointed out that Oxford had a school not for "Materials Science" (as other colleges did), but "The Science of Materials." Because Oxford is famous for its dictionary and they insisted that "materials" is not an adjective
Materials science - Wikipedia, the free encyclopedia