Soapstone or slate come to mind.

I found the perfect size soapstone at a salvage yard for my previous house. In my current house, we have limestone because it is common here. What is the common stone in your area?

I would need to see the FP and your space. Often these stones were what ever could be quarried nearby.

@Patricia Colwell Consulting, here's a photo of the fireplace. We bought a salvage mantel, which we will install after we replace the hearth and surround. I would welcome ideas for the surround as well! Thank you!

@remodeling1840, I think soapstone is common here. Definitely limestone as well. We're in Connecticut. The other fireplace has a white stone hearth that looks like marble.

Did you buy yours from an architectural salvage place or was it specifically a stone salvage yard?

One of the most satisfying projects we have done since moving here has been working on the genealogy of the house. Who was the original owner of the property, when was the house built/ expanded, what other families lived here? Wills and deeds at your courthouse can give you the names of the owners, Ancestry.com can show you more detailed info about the families, local genealogy society can help you find the cemeteries, and Newspapers.com can give you actual news stories about the families. Your local library should have subscriptions to Ancestry and Newspapers.com, probably letting you access them from home free during this virus shutdown. I found stories about luncheons, weddings, receptions, and funerals held here.

they make hearths ready to go in all sorts of natural stone. Look at a paver/rock yard.

here's a Bluestone

or how about basalt?

black slate?

you could stick w/period elements