emery board

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A nail file consisting of a strip of cardboard coated with powdered emery.

from The Century Dictionary.

• noun Cardboardpulp mixed with emery-dust and cast in cakes.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An abrasive material made by gluing grains of emery a rigid material. Often used as a nail file.