Argillite ( /ˈɑːrdʒɪlaɪt/) is a fine-grained sedimentary rock composed predominantly of indurated clay particles.

The Belt Supergroup, an assemblage of rocks of late Precambrian (Mesoproterozoic) age, includes thick sequences of argillite, as well as other metamorphosed or semi-metamorphosed mudstones.

[2] Excellent outcrops of deep purple, wine red, red, blue, turquoise, and green argillites of the Belt Supergroup can be seen in Glacier National Park in northwestern Montana and in Wolf Creek Canyon along Interstate 15 in west-central Montana.

Argillite carvings are synonymous with Haida artwork and are one of the few art forms on the Northwest Coast that is the exclusive right of one cultural group.

This artwork has been of high quality and prized around the world since the Haida first began carving it to trade and sell to sailors around 1800.