Slate is formed from layers of sedimentary rock which is made up of clay and volcanic ash. It is this composition which makes it ideal for splitting into large flat ‘sheets’ for roofing and tiles. Slate is particularly suitable as a roofing and paving material as it has an extremely low water absorption index of less than 0.4%. Its low tendency to absorb water also makes it very resistant to frost damage and breakage due to freezing. Quartzite is a metamorphosed state of sandstone, but significantly less porous as the grains have been cemented together under pressure from tectonic compression, giving it a more glassy structure. It typically has sparkling grains and textured swirls where single grains of quartz have recrystallised in layers during this process. In other words, it looks cool!