lining

When a different material covers the inside of something, like the clear membrane on the inner layer of an eggshell, you can call it a lining.

Many of our body parts have linings — the stomach lining, for example — that protect their insides. Clothing often contains a protective lining too, like the satin lining of your formal suit or the cotton lining of a sheer skirt, sewn inside, or a furry or plush lining meant to add warmth. A "silver lining" is a happy, bright aspect found in a dark or sad event: "I know there's a silver lining to your car accident!"