Ballachulish is the best known of all Scottish slate, both in terms of quality and quantity. It can be readily split and is extremely durable lasting well over a hundred years. There were several quarries near the shore of Loch Leven in Argyllshire, the largest of which was East Laroch (NN085582). Production of Ballachulish slate started at the end of the 17th century and rose to a peak in the late 19th century when 15 million slates were produced annually. From the beginning of the 20th century levels of production declined until finally in 1955 the quarries were closed. In 2002 the Scottish Stone Liaison Group extracted blocks of slate for testing from Khartoum Quarry located at (NN084572),the first new Scottish slate in over 50 years. In 2004 this exercise was followed by the extraction of two cores over 40m into the quarry face to assess the resources of slate in the vicinity of the quarry.
Ballachulish slate from the East Laroch and Khartoum quarries is grey-black with a slight sheen. It is coarse-grained, giving the slate a gritty texture. One of the most distinctive characteristics of this type of slate is the strong mineral lineation clearly visible on the surface. Pyrite grains are common and are usually widely dispersed throughout the slate. The smaller grains are subeuhedral, i.e. they have recognizable but imperfect crystal faces, while the larger grains are anhedral, having irregular faces. In addition there are large clusters of pyrite grains concentrated in quartz veins running through the blocks of slate. The slate is very durable due to the higher than average metamorphic grade and the coarseness of the quartz grains. Pyrite grains when present in an euhedral form, are very resistant to weathering.
Not all Ballachulish slate is of the same high quality, in some quarries the pyrite crystals have been altered to a less stable mineral pyrrhotite which is prone to leaching and often fall out leaving a hole.