Slate is somewhat brittle, but doesn't break as easily into sheets as shale, and can't be scratched with a fingernail. Used for blackboards and slate roofing. Phyllite - Heat and pressure can change the clay minerals in shale into microscopic or barely visible mica. The tiny mica particles make phyllite shiny. Schist - With more heat and pressure, larger mica crystals can form, and sometimes clay minerals will be chemically changed into quartz sand.
Schist layers are visible and the rock splits fairly easily into slabs.
Schist is not necessarily any harder than shale, but it does contain different minerals - visible mica, and sometimes quartz sand. Gneiss - Gneiss forms from shale whenever there is enough heat and pressure to turn shale into magma. This process is called granitization. Gneiss is as hard and solid as granite, and sometimes looks just like granite except for the presence of parallel layers of different rock. Stripes made up of these layers are often very contorted.