each alter has a stone with a cryptic clue to each sacrifice item

I"m pretty new to playing, so have only done 1 boss so far (and found the place to summon the next), but both of them also had a visual clue, imo. Where you summon Eikthyr, one of the stones has an elk/deer on top of it, and where you summon the Elder there are sculptures of the ancient seeds on the top of all the stone pillars.
I'm assuming that trend will continue. Cryptic clue text PLUS a visual clue for each.

Usually if you pick up an object and it's not a trophy and you can't craft anything with it, it's a boss summon item.

Eikthyr breaks this because the item is both a trophy and used to craft something. The Queen breaks this because I don't think you can summon her with Dvergr steins, and you do craft her key, out of something that is otherwise useless. (Would be funny if you could use the steins to summon Haldor.) But otherwise...