"Sunday stone" is an artificial stone composed of porous calcium carbonate that formed on the walls of drainage pipes in some coal mines.^[1][2] It is striped due to the differing levels of coal dust produced during the day and night shifts.^[1] In some cases, a broader stripe is generated every seven days due to active mining not taking place on Sunday.^[1] Broader stripes would also appear when work stopped for other reasons.^[2] In one case, a broad stripe coincided with workers being given the day off to view a cockfight.^[2] Breaks of more than a day would produce an even broader stripe, such as when Christmas fell on a Monday.^[2]

The Sunday stone would over time block the drainage pipes necessitating their replacement.^[1] Where mines have improved ventilation, reducing the level of coal dust in the air, Sunday stone has ceased to be formed.^[1]