Book VII · 15
Whatever any one does or says, I must be good; just as if the gold, or the emerald, or the purple, were always saying this. Whatever any one does or says, I must be emerald and keep my color.
Let any say or do what he will, I must for my part be good. So might the emerald—or gold or purple—never tire of repeating, Whatever any one shall do or say, I must be an emerald and keep my colour.
Marcus Aurelius, Meditations VII.15. Translated by George Long (1862). The Stoic Library.