Holystone:

A holystone is a block of sandstone used when scrubbing wooden decks on
sailing vessels. Large pieces of stone were called "bibles" and smaller
blocks were called "prayer books".

While there is some debate, the term holystone refers to the Sunday
practice of cleaning the ship prior to services. Using a holystone
required the sailors to work on their hands and knees, thus the "prayer"
aspect. The term may also apply to the fact that holystone (sandstone)
was often made by taking gravestone markers as raw material.

Depending on the type of deck wood, holystones leave a deep, clean sheen
and bright luster.