encrustation
n. 结壳; 用覆盖物; 镶嵌; 硬壳
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