Slate

Its defining property is cleavage: metamorphism aligned the clay minerals so the rock splits into sheets a few millimetres thick, which is what makes a roofing slate possible and is nothing to do with the original sedimentary bedding. It is essentially waterproof, with under 0.4% absorption, so a good slate roof lasts over a century. Snooker tables use slate beds because a 25 mm slab can be ground flat to a fraction of a millimetre and will not warp with humidity.

Properties

Density

2,750 kg/m³ (2,650–2,900) — 2.75 g/cm³

Tensile strength

10–30 MPa

Young's modulus

60 GPa

Thermal conductivity

2 W/(m·K)

Hardness

3.5 Mohs

Thermal expansion

10 µm/(m·K)

Floats in water

No

Magnetic

No

What it is used for

• Roofing tiles
• Floor tiles and worktops
• Snooker and billiard table beds
• Electrical switchboards (historically)

How it compares

Does it rust?

Slate contains no iron, so it cannot rust. Whether it degrades depends on the material, not on oxidation.

Is it magnetic?

Slate is not a metal and has no ferromagnetic response.