Why is a carat diamond dull?

Because there is a layer of grease on the surface.

Diamonds are lipophilic and extremely greasy, especially in summer, when the weather is hot and the oil secretion is strong, cosmetics, sunscreens, lotions, creams, etc. Even kitchen fumes can tarnish the ring because of oil. Too much grease will form a film-like substance on the diamond surface, making the original dazzling one-carat diamond ring look gray. If not cleaned, it will gradually damage the diamond surface. Be sure to clean the diamond ring in time.