Crossword Genius

Something hard to value highly? (3)

Ross

I believe the answer is:

gem

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'value highly?' is the definition.
The answer and definition can be both related to property as well as being singular nouns.
Maybe there's a link between them I don't understand?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for gem that I've seen before include "A precious stone like a diamond or emerald etc" , "A jewel of an Orangeman" , "Diamond, ruby, or emerald" , "perfect specimen" , "Valuable stone" .)

I've seen this clue in the Evening Standard.
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