In front of it the King of the people of the Mountains (most probably the local population of the Vale of the river Morthond and the mountain EredNimrais) swore allegiance to Isildur taking an oath to be an ally of Gondor against Sauron. (see Oathbreakers)

The Stone of Erech itself, however, is a mysterious item. Could it be a Palantír?

It is known that the Seven Seeing Stones brought by the ships of the Faithful from Numenor were divided, as follows:

Nothing about Erech!

Yet on the other hand, the Stone of Erech was black and round as a globe:

Compare :

And to think of it, what other stone, if not a Palantír, could have been of such a great value to Isildur that he would have worshiped so greatly that he made the King of the people of the Mountains take a most solemn oath precisely in front of it?!

Besides:

What other stone, if not a Palantír, could have Isildur brought to Middle-earth from Númenor? There is no information about any other item of similar features that had been saved from the drowning of the isle, except for the Palantíri.

It is also known that some of the Palantíri were of a very large size and the Stone of Erech was a very large one indeed.

So, even if it is particularly said that the Faithful had saved and brought to Middle-earth no more than seven Palantíri, the stone placed on Erech resembles so much in features and importance a Seeing Stone, that one might think of it as an eighth Palantír, not mentioned in the story, or just forgotten to be developed into a story.

Could it be?

tbc