Lithification usually involves compaction by overburden pressure, followed by cementation by some other mineral that fills in the gaps, transported by water. Most fossils, say, a dinoaur bone, have had their organic molecules replaced over the eons by other minerals. Toward the higher grades, metamorphic rocks are hard to distinguish from igneous rocks because much recrystallization may have taken place. Gneiss has large crystals like granite but is almost always banded. Schist is layered and often flaky, but has smaller crystal sizes.
Slate is low-grade metamorphic shale.
Its tiny grains are squashed together and it is more brittle and breaks in sheets more readily than it sedimentary cousin.