Translation:The woman's jewellery is expensive.
we would normally talk about jewellery [ even if it's only one item] not a jewel
Yes, jewel in the singular refers to one stone "India was the jewel in the crown of the British Empire".
I said, "piece of jewellery," to try to distinguish it from the pural κοσμήματα. I wondered whether just saying "jewellery" might inappropriately suggest that all of the woman's jewellery is expensive, when only one particular item was being referred to. Am I overthinking it?