What is my fireplace floor made of, and how do I repair it?

[Glaseddonut]

I've been demoing my hearth and surround in my recently bought 1955 house in NC.

Everything has been floated in thick set mortar, and I noticed as I was demoing the hearth, some small cracks and chips in the fireplace floor itself (which I wasn't planning to demo).

Inspecting it closer with my hand, I was able to pull of large chunks easily! It appears to be a large black piece adhered to the same thick set mortar as everything else, not tiles or bricks.

Any idea of what it is and how to safely repair it for fires again?!

 

[Sodbuster]

I would use refractory mortar and firebrick.

 

[bholler]

Yup sodbuster is right firebrick and refractory mortar. Keep the mortar joints under 1/8" for the best durability. I generally use about 1/16" joint.