Ok so i took off my cap from the radiator and there was this brown rock looking thing it was hard substance such as dirt smashed together what is this?

Sounds like some kind of stop leak product was used. I would recommend a coolant flush.

probably rust.
mostly caused from running straight water.

1. What year XJ?

2. Do you know the history of routine cooling system maintenance?

3. Any stop leak product ever used?

+1, rust.

+1 rust......

Post a pic

Yes, we like photographic evidence of this "hard brown", living in your radiator...

Maybe it's a piece of a bar from Barsleaks.....