Slate is popular for a wide variety of uses such as roofing, flooring, and flagging because of its durability and attractive appearance.
When selecting slate, you can use the idea of the metamorphic spectrum to help you identify stones that are suitable for your needs. Low-grade slate, which has not been subjected to much metamorphism, may split along its layers. Higher grades of slate are more robust, and phyllite is also strong and resistant to cracking. Schist can sometimes be troublesome because the larger mica grains are prone to peeling off. However, some schists have less mica and more quartz and are exceptionally strong. Your best bet for evaluating a given stone is to examine the edges and look for signs of flaking. Also, note how much sparkle the stone has. A bit of shine is an indication of a stronger slate.
While slate was originally used mostly in the Northeastern US due to the proximity of slate quarries, slate’s popularity has spread nationwide. Quarries in the US still operate, and slate from India, Asia, and South America have also become commonplace. (from Use Natural Stone website: usenaturalstone.org)