Crossword Genius

System of weights used for precious metals and gemstones (4)

Ross

I believe the answer is:

troy

(Other definitions for troy that I've seen before include "City besieged by the Greeks - system of gem weight measurement" , "The pound - had twelve ounces, ancient city" , "Ancient city besieged to free beautiful Helen" , "Ancient city; type of weight" , "System of weights introduced by French" .)

I've seen this clue in The Mirror, The Irish Times and The Guardian.
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