The significance of the stone must be that Peter Jackson thought it looked cool. There is no mention of it in the book.

Treebeard tells Merry and Pippin that the Entmoot meets at Derndingle (the name is archaic English, meaning "secret valley"). 

>‘Where is Entmoot?’ Pippin ventured to ask.

>‘Hoo, eh? Entmoot?’ said Treebeard, turning round. ‘It is not a place, it is a gathering of Ents – which does not often happen nowadays. But I have managed to make a fair number promise to come. We shall meet in the place where we have always met: Derndingle Men call it. It is away south from here. We must be there before noon.’

><sub>*The Two Towers: Treebeard*</sub>

When they arrive at Derndingle, we read:

>The hobbits saw that they were descending into a great dingle, almost as round as a bowl, very wide and deep, crowned at the rim with the high dark evergreen hedge. It was smooth and grassclad inside, and there were no trees except three very tall and beautiful silver-birches that stood at the bottom of the bowl.

><sub>*The Two Towers: Treebeard*</sub>

No stone is mentioned, although Jackson could argue that was just an oversight.