The old ring was encrusted with dirt, barely showing its original shape.

encrusted 是动词 encrust 的过去分词形式。encrust 由前缀 en-（表示"使进入某种状态"）和 crust（"硬壳、外层"）组成，字面意思是"形成硬壳状覆盖物"。在句中表示"被厚厚的污垢覆盖"的状态。