As a native English speaker, I would expect "jewel" to be given an adjective of some sort. Such as; The girl is wearing a cheap/expensive/old/colorful jewel.
Otherwise, I would say; The girl is wearing jewelry.
Which is a very good observation, actually. The case is the same in Greek as well (meaning that, you'd either say ακριβό/φτηνό/μεγάλο etc. κόσμημα, or just κοσμήματα.) We'll keep this one in mind for the new tree, thank you. ^.^