millstone

noun3

1. •
   A large round stone used for grinding grain.
2. •
   (figurative) Ellipsis of millstone around one's neck, a heavy responsibility that is difficult to bear (referring to Matthew 18:6 in the Bible).

   • Paying the mortgage every month is a millstone round their necks.
3. •
   A coarse-grained sandstone used for making such stones; millstone grit.