The gem cannot be polished without friction nor man without trials. Confucius
Have you ever really had a teacher? One who saw you as a raw but precious thing, a jewel that, with wisdom, could be polished to a proud shine? Mitch Albom
A gem cannot be polished without friction, nor a man perfected without trials. Seneca
The diamond cannot be polished without friction, nor the man perfected without trials. Chinese Proverb