In a field on the other side of the house is another monument to a hero of that day, to the memory of the brave young Seward, who fell, slain on the spot by Macbeth. A stupendous stone marks the place; twelve feet high above ground, and eighteen feet and a half in girth in the thickest place. The quantity below the surface of the earth only two feet eight inches; the weight, on accurate computation, amounts to twenty tons; yet I have been assured that no stone of this species is to be found within twenty miles.
From ‘A Tour In Scotland and Voyage to the Hebrides, 1772’ by Thomas Pennant (1776).
Odd one this Rhiannon , Sewards stone is usually considered to be a nearby much smaller stone than in the description , the description fits Mabeth's stone whilst omitting mention of the cup marks .
Hmm that is true about the size, you're quite right (most unobservant of me). So the name has got moved? Or Pennant got confused (understandably). Maybe it's just that once you've got a good story going, you have to explain everything in the vicinity according to that story, so you end up with Seward's Stone and Mrs Macbeth's stone and all sorts? If you look at Pennant's book he goes on for pages and pages about all these places in the landscape that have their place in the whole Macbeth story.
It may tie in with the fact that Belmont was the home of the Lord privy seal it and happened to have a few genuine prehistoric monuments , (a couple of standing stones and cairns ,).The Holinshedian/Shakesperian and real Macbeth was located not too far away (although he wasn't killed there) .