Encrustment is a word used to describe the process of forming a hard outer layer over a surface. This term is often used in geology to describe the formation of minerals on rocks or fossils. Encrustment can also refer to the accumulation of dirt or other debris on surfaces such as floors or walls. Synonyms for encrustment include coating, crust, covering, layer, and plaque. These words are often used interchangeably to describe the appearance of a surface that has been covered or overlaid with a hard substance. Other related words include calcification, petrification, and mineralization, which describe the process of turning organic material into stone.