Argillite is similar to the substance known as catlinite which was used by the indigenous peoples of the American Plains to carve their ceremonial pipes. However, while catlinite is of a reddish-brown color because of its high iron oxide content, argillite is a dark-grey to black color because of its higher carbon content.

Argillite used by the Haida is found at the Slatechuck argillite quarry on Slatechuck Mountain near Skidegate on the east flank of Graham Island of Haida Gwaii. In the Skidegate dialect of the Haida language argillite is called kwawhlhal. The quarry is very difficult to get to and its exact location is a heavily guarded secret.

Argillite is an important commodity for the Haida people and presently only the Haida have the right to use the variety found at Slatechuck Creek, which occurs nowhere else in the world. One of the first documented reports of the Slatechuck quarry occur in about 1820 when prospectors looking for coal or copper came across a large deposit of the black slate.

A metamorphic rock formed from petilic sediments, this decorative material became renown because of the imaginative and skillful carvings of the Haida Indians of Skidegate and Massett, of the Queen Charlottes. Another source for Argillite is north of Horseshoe Bay.