Material in sapphires and corundum (7)

I believe the answer is:

alumina

(Other definitions for alumina that I've seen before include "Metal oxide found in many clays" , "Metallic oxide that is a constituent of many clays" , "Mineral in many clays" , "Abrasive" , "Compound in many clays" .)

I've seen this clue in The Times.

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