It is an irregular slab of the local Old Red Sandstone, certainly broken in two. The Triple-Oval symbol falls off the "RH" side, so it is possible there was once more of it. I call it RH side, because I am following the convention of treating it as depicted in ECMS, ie circular symbol to the left, and the Triple-Oval to the right. There is no way of knowing whether it was designed to lie flat as found, or upright, and if upright which symbol should be uppermost. On the other hand, the orientation of the Triple Oval implies that it should at the very least lie on its side, if not actually prone. It may have been designed as part of a coffin cover. Other stones have been recovered from such positions, and it is possible that was their intended use, rather than a re-use at a later date. I was tempted to say it was unlikely to have been conceived as a flooring slab, but later generations have put considerably more effort into carving graveslabs, only to lay them as paving that becomes totally eroded with traffic over them.

The Symbols.

The stone has only two symbols on it, both single-line incised.
[1] There is a circular one with five semi-circles arranged around a circle in the centre, not unlike a stylized flower. It has a further ovalish figure attached to its RH side, and this in turn has an oval within it, where the two other figures meet. It creates an overall impression of an old-fashioned fob watch, with a decorated case, and its winder and loop to hang it by.
Consensus of opinion has it being an unfinished Triple-Disc symbol. Another viewpoint is that it could be a Mirror symbol.
It is one of those annoying mysteries of Pictish studies.
If it is a Triple-Disc symbol, why is it unfinished? Or was it completed, and the other side has just flaked off?
If it is a Mirror, where is its handle?
Is it perhaps a symbol in its own right?
[2] The Triple Oval is incomplete, but that is merely due to flaking of the RH side and top.

[1] As a Triple Disc, the first symbol is relatively rare, being the only representative of it in the area, although there are three others further North on the same coast, out of a total 14. Interestingly there are 3 of them grouped close together in Angus, and two of these at Glamis (see below) ie Glamis 1 and 2.
As a Mirror, it would be the only one in this area, the bulk of them appearing much further South, although there are three in Orkney, and even one in Shetland.
[2] The Triple Oval is a rare symbol on stones. To date, there are three from this district, one from close to Inverness, and a fifth from Glamis, (Glamis 4) but it is only a fragment so it is impossible to know what other signs it might be combined with, unless other fragments that match it turn up. Who knows what's in that rockery at Glamis?

Glamis 1 and 2 both have both the Triple Disc symbol, and the Mirror without Comb, and are complete as they are, but is there a possibility that there is a stone in the vicinity with a Triple Disc and Triple Oval, or is the Birkle Hills Stone destined to have a unique combination of symbols?