Proper way to make shapes adjectival? Studying shapes of this article: < And noticed only some shapes have adjectival forms. What is the proper way of saying stuff like Octagonal? I'm thinking either ShapeNoun, ShapeNoun, or possibly even ShapeNoun but this one feels wrong.

Yes, only a few words ( and ) have i-adjective forms. For others, you can treat them as no-adjectives: (octagonal), (star-shaped), (cylindrical) and so on. (Don't forget ; would mean "starry".)

Also note that and are suitable in everyday conversations but these are not suitable in mathematical contexts. See this answer. The "technical" version for () is / ("circle/circular") or / ("sphere/spherical").

and would mean "octagon-ish" (i.e., something not exactly octagonal).