How does slate form?
Slate is a foliated metamorphic rock formed from shale under relatively low heat and pressure — the mildest, first step of metamorphism shale can undergo.
How can you identify slate?
Use several clues together.
One photograph shows one specimen, not every variation.
Similar at first glance.
Compare the materials side by side.
Photographs are not shown to the same scale.
Slate ↗
Splits into thin, flat, smooth sheets — even more cleanly than the shale it came from
Shale ↗
Very fine-grained, smooth to the touch
Shale, its parent rock — slate's sheets are flatter, smoother, and split more cleanly, and it has a subtle sheen shale lacks.
Where it’s found.
Regions with ancient mountain-building/tectonic compression — historically quarried heavily in Wales and the northeastern U.S. for roofing tiles.
More from this family.
Phyllite
Distinctive silky/satiny sheen (from the small aligned mica crystals) — this is the fastest way to identify it
Schist
Visible, large, sparkly mica flakes (or other platy minerals) aligned in wavy layers
Gneiss
Bold, coarse banding, alternating light (quartz/feldspar) and dark (mica/hornblende) layers