The term 'LAYER OF MINERAL' can be solved by the crossword solution SEAM, which is a four-letter word. A SEAM is a thin layer of mineral that is found between two larger layers of rock or sediment. Seams are often formed when two different types of rock or sediment are pressed together and form a thin layer between them. Seams can also be formed when minerals are deposited in a thin layer on top of existing rock or sediment. Seams are important to geologists because they can provide information about the history of the rocks and sediments in which they are found.