Bullauns, or cursing stones are common in Ireland, but until the news today they've not been known in Scotland:
Dating from about 800 AD, the stones are associated with early Christian crosses - of which there is one on the isle.
It was found in an old graveyard by a National Trust for Scotland (NTS) farm manager.
The stone is about 25cm in diameter and engraved with an early Christian cross.
It was later found to fit exactly into a large rectangular stone with a worn hole which was located at the base of the Canna cross.
You can read more about bullauns here (lots of photos, although I have to say I'm not sure about the claims that these things date as far back as the megaliths - not something that's easy to prove, but I guess they do seem similar to cup and ring marks so there is logic to it; as the article says, though, most are seen as early Christian, although that doesn't mean to say that some couldn't predate that). E. Estyn Evans has a little bit about them as well:
"Smooth pebbles resting in certain stone basins are turned three times against the sun ... Their utilization as cursing stones continued into recent times. I was told of the example illustrated at Killinagh in Co. Cavan, that 'you would think twice before turning the stones, because the curse would come back on you unless the cause was just'."
Irish Folk Ways, 1957, p299-300.
The tuathal (anti-clockwise; against the direction of the sun) direction indicates their purpose for cursing, a sign of negative intent, and given the caveat about it possibly rebounding on you, you'd have to be pretty damn pissed off at someone to use them.
As bullauns are for cursing there are other kinds of stones associated with healing or blessing. Some of them are rounded depressions in rocks, and the water that collects in the depression can be used for healing - curing warts or infertility, for example (Evans references an example of these in Cairngorm, Scotland). There are also healing stones that were commonly kept at wells, and borrowed by the locals as needed:
"At St. Olan's Well, Dromatimore, Co. Cork, the rounds include visits to the saint's Cap and Stone. The former is an oval quartzite stone which rests on an ogham-inscribed monolith and which replaces one, removed by the parish priest a century ago, which was invested with magical properties. 'It was said to be an unfailing talisman, and was much sought after for various feminine ailments, particularly maternity cases. If worn on the head and carried three times round the church it was said to cure the most violent headaches and, in addition, it had the gift of locomotion in that, if removed to any distance, it unfailingly returned to its original position.' "
Irish Folk Ways, 1957, p299-300.
In Scotland, F. Marian McNeill details a lot of healing stones that were kept by certain families for similar purposes.
Given the close historical links between Scotland and Ireland the find of this bullaun is not all that surprising - aside from the fact that none have been found before, perhaps. There are more than likely other examples to be found across Scotland; as Dr Forsyth notes in the BBC article, there are plenty of examples of the base stones with depressions in them, which could be bullauns with the stone missing, perhaps, or else examples of the 'wells' that Evans describes. Or otherwise they might be gruagach stones, where offerings of milk were left in the depression to ensure the local gruagach would continue to watch over the cattle...
The find of this bullaun just goes to show how much there is for us to still find out, though, and that's what makes it exciting for me. What's next?