encrustation
n. 結殼; 用覆蓋物; 鑲嵌; 硬殼
An encrustation is a hard and thick layer on the surface of something that has built up over a long period of time.
1. the formation of a crust
2. a hard outer layer that covers something
3. a decorative coating of contrasting material that is applied to a surface as an inlay or overlay