Claystone

About it

Mudrock made mostly of clay-sized grains, smooth and soft enough to scratch with a knife, with no grit at all. Some kinds soften or crumble when soaked; common in red-bed sequences and around coal seams.

Type

Rock (sedimentary)

Group

Sedimentary (clastic)

Lustre

earthy, dull

Colours

grey, red, brown, green, cream

Transparency

opaque

Cleavage and fracture

blocky to conchoidal, not fissile

Typical values: real specimens vary. For more, see mindat.org.