Holystone is a soft and brittle sandstone that was formerly used for scouring and whitening the wooden decks of ships. It was used in the British and American Navy for scrubbing the decks of sailing ships. The term may have come from the fact that 'holystoning the deck' was originally done on one's knees, as in prayer. In realistic reference to their size, smaller holystones were called "prayer books" and larger ones "Bibles"; also, a widely quoted legend attributes the name "holystone" to the story that such pieces of stone were taken for use from St. Nicholas Church in Great Yarmouth. More plausible is the use of stones taken from the ruined church of St Helens, Isle of Wight; tall ships would often anchor in St Helens Roads (the strip of water immediately adjacent to St Helens) and take provisions and fresh water from St Helens before setting off on their journeys. Read more on Wikipedia
Lowry refers to the term in his novel Ultramarine; "Well, those were the ancient violences, the old heroic days of holystones; and they are gone, you say." Ultramarine Pg. 47. This use of the term is from Ocean Steamships; "In the middle watches the decks are scrubbed with sand and brooms and brushes, for the old, heroic days of holy-stones are over, and a hundred pounds of effort are no longer expended for an ounce of result." (Pg. 166). Lowry also refers to the term in his poem 'No Kraken Shall Be Found Till Sought By Name'; "Here is the ship then, with decks all holy white/Pure as the stone that scrubbed them to the bone. ( Collected Poetry Pg. 198). Lowry would have also been familiar with the phrase which features in J. Johnston Abraham's The Surgeon's Log (Pg. 191) - a book Lowry refers to in the short story 'Enter One In Sumptuous Armour'. (Psalms Pg. 231).