A holystone was a square of pumice or sandstone that along with a bit of sand would be used to bring up wooden decks clean and white.

The block was about the size of a bible and as cleanliness was close to Godliness....

Usually on a wooden ship this would be carried out during the latter part of the morning watch (4 - 8 a.m.) or the earlier part of the forenoon watch (8-12 midday) and origianlly it was done on hands and knees.