The Standing Stone

[Priya]

What was the significance/purpose of the ‘stone’ at the center of the hollow in the Barrow-downs adventure?

“… they … went down into the hollow circle. In the midst of it stood a single stone, standing tall under the sun above, and at this hour casting no shadow. It was shapeless and yet significant: like a landmark or a guarding finger, or more like a warning.”
- Fog on the Barrow-downs

I presume Tom referenced it when at his abode:

“Don’t you go a-meddling with old stone or cold Wights …”.
- In the House of Tom Bombadil

and in poetry:

“… The cold stone is fallen; …”
- Fog on the Barrow-downs

I have my own theory about it - but what do others think?