。
Hardship polishes one into a gem.
Literal
Hardship you [object-を] jewel-to-make.
A famous four-character (technically five-character) Japanese saying: 艱難汝を玉にす — 'hardship turns you into a jewel', i.e. adversity builds character. 艱難 (かんなん) is a literary compound for 'hardship, tribulation'. 汝 (なんじ) is an archaic pronoun for 'you'. にす is a classical contraction of にする ('to make into'). The whole phrasing is deliberately classical/elevated.