expression
a diamond on a dunghill is still a diamond; quality is quality
Proverb meaning that something of high quality retains its value even when damaged or in poor condition. Literally 'even if it's rotten, it's still sea bream'.
彼は引退したが、腐っても鯛で、まだその技術は一流だ。
He's retired, but a diamond on a dunghill is still a diamond; his skills are still top-notch.
From the idea that sea bream (鯛), a prized fish in Japan, is still valuable even when rotten, implying that inherent quality persists despite outward decay.