'Rough diamonds' are what we call people in whom the pressures of life have developed great strength and character without making them uncomfortably perfect.

What does 'them' here refer to? 'People' or 'great strenght and character'?

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Good! Just as I thought.

Thanks, Jim!
I've normally heard such a person described as "a diamond in the rough". That sounds better to me, but perhaps only because that's the way I've heard it.