Which part of the fireplace are you referring to specifically? The shape and material of the fireplace or the pile of burning embers?
For the fireplace itself, I can't see the top, but it looks like 3 full concrete blocks for the base with quarter blocks for the sides. I'm sure you're talking about the embers, though. There are hundreds of hidden blocks in the creative menu that you can make visible by clicking the DEV icon (I think it's the 2nd or 3rd from the left. just hover over, it tells you) Once that's enabled you should see a bunch more blocks appear. I'm not sure what category this will be in, but I'm sure if you just search for fire or ember, it'll appear.