The Stone of Erech, also known as Black Stone, was the only standing stone where the king of the mountains swore an oath to Isildur. It was depicted as being large, having possessing some six feet in diameter and seemed to be spherical in shape -- half of it was underground at the time of the oath. The other half that was visible/above ground was made out to be as tall as a man. It was located on the Hill of Erech.
The hill of Erech was located in Gondor, at the beginning of the Blackroot Vale in the White Mountains (Ered Nimrais). The head of the Blackroot Vale was where the entrance to the Paths of the Dead was located. The stone was brought to the hill of Erech by Isildur, who rescued it from the sinking of Númenor.
In the very early days of Gondor, the king of the men who lived in the White Mountains swore an oath of allegiance to Isildur on the Stone of Erech. This oath was betrayed when Isildur called upon them to fight Sauron in the War of the Last Alliance. That caused Isildur to curse them to remain undead until they had fulfilled their oath. They haunted the Paths of the Dead for more than 3000 years.
It was on the Stone of Erech that Aragorn called on the King of the Dead, and the other accursed, to fulfill their oaths so that they could rest in peace. This occurred at midnight. The Oathbreakers helped Aragorn take over the ships of Sauron's allies from the south. They were finally put to rest by Aragorn for fulfilling their oath where they had not done so before.
If the stone were to be truly spherical, it would have to have been at least six feet tall. Since half of it would have been between 5'5 and 6'2 (as given from the text stating it was "as tall as a man"), it should have been three feet in radius. Therefore, if half of it was buried, it should have emerged from the ground to a height of around three feet. Alternatively, if the stone extended above the ground by six feet, its radius would be six feet, and its diameter twelve feet, with half of it underground, and half of it above ground, thus only making the situation seem logically assumptive.
Appearances in adaptations
The Lord of the Rings Online is only adaptations to date to depict the Stone of Erech in any form.