Assay subjects (4)

I believe the answer is:

ores

'assay subjects' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for ores that I've seen before include "Rocks or minerals containing metals" , "Mined minerals" , "Metal-bearing minerals" , "Mineral deposits" , "Bauxite, haematite etc" .)

I've seen this clue in The Wall Street Journal.

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