Because of its massive weight and the resulting foundation requirements, stone is seldom used today as a structural building material. It is, however, still widely used as a surfacing or facing in walls, floors, steps, walks, paths, and roads. The strength of stone depends on its structure, the hardness of its particles, and how those particles are held together. Hardness of stone is important only in horizontal applications such as flooring and paving. But hardness does influence how difficult (and costly) the stone is to saw, shape, dress, or carve. Both strength and hardness are proportional to silica content. Workability decreases as the percentage of siliceous materials increases. The resistance of a stone to wear and weathering also depends on its silica content.