The top side of observers closely matches the color of polished basalt.
I have used it here to make a rosary. For greater unity of color, the bottom side of pistons are used as tiles. Variety is added by having furnaces for lighter tiles.
This technique does not work sideways. The arrows on the observer can only be made to face up/down and only on the north/south sides. So, when planning a facade the direction matters.













