How to Identify Slate: What to Look For and Home Tests

Metamorphic rocks were changed by heat and pressure. Look for a sugary or platy texture, minerals lined up in one direction, and a hardness or ring that the parent rock did not have. The properties that identify Slate — luster: Higher luster than shale; color: Usually gray or black; cleavage or fracture: Slaty cleavage; splits into thin slabs or sheets along parallel planes. For a rock, texture and grain size matter more than any single hardness number.

• Cleavage test: slate's 'slaty cleavage' — minerals aligned perpendicular to the pressure direction — lets it split along regular, parallel planes more evenly than shale.

• [Shale](/lookup/rocks-and-minerals/shale): Slate is harder, denser, and higher-luster, and splits more evenly; shale is softer, splits less evenly, and may feel slippery when wet.

Formed by recrystallization of Cambrian-to-Ordovician fine-grained sedimentary rocks (such as shale) under low-temperature, high-pressure conditions.

Photograph a fresh, unweathered surface in daylight, then a second shot of any crystal faces or broken edge. Include a coin for scale. If you did a streak test, photograph the streak next to the specimen. The app suggests the likely mineral or rock; confirm with hardness and the tests above before you label it.

This entry is drawn from the Vermont Geological Survey's 'Vermont State Rock' page. The Slate Valley on the Vermont–New York border is a well-known slate-producing region in eastern North America.