Slips on stone floors injure countless people every year, but picking a better floor isn’t easy. So in the most comprehensive study of its kind, researchers set out to figure out which flooring stones are the slipperiest. The team tested 81 types of commercial paving stone—from 13 different rock families—and seven different surface finishes (hammered, flamed, brushed, saw-cut, honed, polished, and sand-blasted) under both wet and dry conditions. The results showed, unsurprisingly, that stones are slipperier when wet. In contrast to some previous studies, however, the researchers found that slipperiness was not always determined by the roughness of the stone’s finish. Instead, the stone’s composition plays an important role, the team reports this month in the Journal of Materials in Civil Engineering. The more quartz in the stone, the less slippery it was. Micha schist floor stones, for example, which have one of the highest quartz contents the team tested, were also among the least slippery generally. But there were a few exceptions. Travertine, for example—a form of limestone deposited by hot springs—is low in quartz, but compensates with its high porosity, which can add roughness to more uniform finishes. A similar effect is seen with sandstone. In contrast, granite has a high quartz content but is very slippery in wet conditions, because the other minerals that typically make up the rock are easily worn down during the abrasive finishing process. The stone’s composition is the most important when dealing with so-called honed finishes—popular for their appearance and resistance to wear—which are on the cusp between being too slippery (as with the smoother polished finish) and always safe (as with, for example, the rougher sawn and sand-blasted finishes) in wet conditions. To be safe, the team says, quartz-rich stones other than granite are a good choice generally, but one should double check that particular stone-finish pairings work well together.