It is because gold is rare that gilding has been invented, which, without having its solidity, has all its brilliance. — Thus, to replace the kindness we lack, we have devised politeness, which has all its appearance.

[C’est parce que l’or est rare que l’on a inventé la dorure, qui, sans en avoir la solidité, en a tout le brillant. Ainsi, pour remplacer la bonté qui nous manque, nous avons imaginé la politesse, qui en a toutes les apparences.]

Pierre-Marc-Gaston de Lévis (1764-1830) French noble, politician, author, aphorist

Maximes et Essais sur Différents Sujets, “Pensées Détachées,” # 180 (1808)
    (Source)

(Source (French)). Frequently misattributed to his more famous father, Francis de Gaston, first Duke de Lévis.

The full aphorism also includes a final clause, "et au défaut de vertu, nous avons l'honneur, qui en a l'éclat" ("and, in default of virtue, we have honor, which has its luster").

The French was incorporated in standard French grammar books for many years.

The English translation shows up in several cases without any attribution and in varied contexts (1, 2, 3).