"A beautiful piece of jewelry."
Why is this question in this lesson? Also thr word by word translations are very misleading here.
You can't use חתיכה to mean an item. It literally means a piece of something that was once whole (like a piece of cake)
The general phrase an english speaker would say for one specific item of jewelry could be "a piece of jewelry". So that phrase would represent a single noun. So together with an adjective, you only need the single form of the noun follwed by the adjective. Any fluent Hebrew speakers agree?