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.