good egg

Seems to fit this from that site:

The 'diamond' part of the name not only comes from the lovely Harlequin diamond pattern on the trews it also comes from the use of the word as a part of the term 'diamond in the rough' meaning one having exceptionally good qualities or the potential for greatness but lacking polish and refinement.

But your explanation puts it all into some context that helps me understand. It's kind of like the Yiddish mensh, yes?