The stone is an unsmoothed natural slab of black whinstone with quartz veins running through it. The symbols are incised directly into the rough surface. Whinstone is an extremely hard stone, much harder to work than sandstone.
It is set on a low cairn about 6ft (1.9m) in diameter. Charles Elphinstone Dalrymple found this out when he was excavating around the stone in 1856.
He also found a rectangular grave about 4½ft (1.6m) down, at the southern perimeter. The grave was roughy 7ft (2.2m) long and it was orientated E-W, but it was apparently empty.

The symbols are as follows:-

1. Right at the top, a Double-disc and Z-rod lying horizontally.
2. A Serpent and Z-rod also lying horizontally.
3. What appears to be a Mirror, below that again.

The Z-rod through the Double-disc is of the usual type, having an "arrow-head" at the top and a floriated disc with a dimple in it at the lower end., but the Z-rod through the Serpent has a floriated disc at both ends.
Note that the Z-rods are roughly at right-angles to one another. There is to be an essay on the significance (or otherwise ) of the orientation of symbols. It might be of no importance whatsoever, since it often looks as though the symbols are just squeezed onto the stone wherever they fit. The face here dictates a single vertical column.
The Mirror is, like the Lindores Stone, unaccompanied by the usual Comb.
However, if you can get hold of a copy of the ECMS, either the original, or the Pinkfoot version of it, and turn to the photo of the Picardy Stone, there is what appears to be a series of pockmarks, extending down from the RH edge of the lower "ball" of the handle, which create a semi-circle of very similar diameter to the full circle above. The LH edge has a curving line leading off from it in an arc that mimics the full circle above. Unfortunately, the area between the end of the pockmarks, and the bottom of the other surmised line is very irregular.
Is it a trick of the lighting, (which is always a problem with such worn, rough-surfaced stones) or does it represent an unfinished version of a Double-disc?*
As it stands, it represents a mirror of the late Roman Iron Age, a polished bronze disc set into a handle. Note it has no rim, which means it must have been rivetted directly into the handle, unlike the Lindores mirror which has a distinct rim around it.

*The thinking here is that they probably pecked the line first, and then chiselled out the spaces between them, before deepening and smoothing the groove to give a more consistent line. Anyone trying to draw a free-hand circle of any size will be aware of the variety of shapes other than a true circle you can achieve, if that's the right word for it. Now, try to imagine giving up the relatively easy-moving pencil on soft paper for a chisel on stone that is sometimes used for gravel for bottoming in road construction.