Can somebody explain this rock, found on the island of Kerrera, in western Scotland? The material is like soft slate. There used to be a cube of pyrites in the square hole at the centre of each circular part.
Kerrera is composed of: volcanics, the Old Red Sandstone, and metamorphics (mainly slates I would guess).
The slates would be the source of the pyrites.
I am tempted to suggest that maybe your specimen was a lump of old metal mining tailings (settled-out mining waste) that had included cubes of pyrites. But as far as I know, the only mining in the area is/was slate. (I did some of my postgrad research in the area, camping on the Garvellachs; I saw slate quarrying on the island of Seil, if I recall correctly.)
The circles (sections through spheres/spheroids) are undoubtedly alteration haloes - the pyrites affecting the chemistry/mineralogy nearby. The exact nature of the haloes, I don't know.
The best I can suggest is that it is soft slate (slate can become softened due to alteration of the micas), but I am not confident. I think such soft slate would be unusual, and the haloes are not familiar.