From A & D Sutherland Ltd; the Spittal Quarry, Caithness.
The quarry is near Thurso, Scotland. Caithness flagstone is quarried in 16 principle layers split into a total of 49 individual flagstones, which range from 20mm and 80mm approximately. Traditionally the stone is worked by splitting it into flags by hand; the resulting flags have been shipped all over the world.
A set of bronze age stones in Caithness flag are still standing at Achavanich.
Caithness flagstone is from the Old Red Sandstone of Devonian age (BRE, British Stone List).
It's hard, compact, fine grained, and dark grey in colour - weathering turns this paler over time.