Although the picture for this item shows knobs for two concentric pots and it is described as a "Set of Potentiometer Knobs" and "Quantity: 2 potentiometer knobs", it is only a single stack (or at least, that is all I was sent) with one top part and one ring part.
It's smaller than most concentric knobs and won't fit on concentric guitar pots with more common shaft widths - be sure to check the measurements. The ebony is a little bland, just pure black without any grain or natural brown lines through it, which is more or less as shown to be fair.