Go to a granite store (marble would work too) and find a remnant that goes with your décor. We had a brick hearth that we hated and took them our measurements and they had a piece left over from someone elses job. All they charged us was to polish the edges and installation. We had two done and I think the whole job was like $400. That crack would be impossible to repair.

This happened to our hearth in our mountain cabin when one of the men in the family decided to split wood on the hearth.............We went to a concrete contractor and had them replace it.......

go simple and place something interesting on the mantle

I agree with the other person that answered, go to a stone fabricator, find a remnant and fit it to your fireplace hearth

Thank you all for responding. If a remnant is the solution, does it replace the concrete or go over it?