Looks like marble to me....late Victorian....nice!

Looks like marble to me, too!

Check it with a magnet; They were very crafty at painting cast iron to look exactly like fine marble (and they had amazing foundry and machining skills). With the inherent weakness of marble, that is no disadvantage, if it looks as good in person as it does on the screen. If it is stone, I would be worried that whatever mechanical fasteners keep the tablet attached to the facing is stressing the material a lot when it is moved.

I restored a 1850's pink marble mantel made up of a handful of pieces, many of which had broken/fragmented, and the only thing that had held it together was a cement of plaster of paris, and the tablet was basically held down by the wall plaster that it was set into. Cast iron does not have any of those weaknesses.

Casey

other red & black stones include mahogany obsidian

Jasper

bauxite

As you can see, all these can be cut and polished. But Sombreuil 's idea seems likely.

This is a pile of pieces of a fireplace surround. Some of this assembly is still held intact by plaster. These are pieces of slate with a faux decoration. They came out of an abandoned house. The decoration is a combination of sponge and marbleizing. If you have a faux decoration on your stone it is in remarkable condition considering the fireplace fluxuated from hot to cold so often. You can easily tell if the decoration is faux, just look at it from the back side, if it is plain you have faux decoration.