I purchased some Grade 28 Insulating Fire Bricks when constructing an indoor electric foundry. Now these aren't cheap costing at the time of purchase £5 each. However they're a specialised item capable of withstanding temperatures of 1500°C / 2732°F.
See the accompanying video here.
Bricks are generally rectangular in shape and to achieve the hexagonal shape I required, I needed to remove two 30° slices from each brick. The company I purchased the blocks from were happy to provide a cutting service at £5 per cut, effectively making each brick £15 each. You can imagine what I told them.
The good news is these bricks are VERY easy to cut. Genuinely if you had the patience you could cut through them with a spoon. They take tools a treat and you can saw, carve and shape them with handsaws, files, chisels, rasps, sandpaper, etc.
With that said, don't use your best tools. Yes cutting them is easy, really easy, but despite the lack of effort involved your sharpest handsaw will soon be your dullest once you start cutting. As such use old, rusty, blunt tools. These will still cut just as easily.
You can also use electric tools. I've used an electric mitre saw and a router on these bricks and you can get a very nice finish BUT it will produce a lot of very hazardous dust. Don't shrug this of as health and safety softness - it should be a very genuine concern to you. Ideally you should wear a quality dust mask whenever you cut these types of bricks, but most especially if you use electric tools. It's all too easy to produce thick clouds of carcinogenic dust.
If you're skilled with hand tools, then stick with them, but personally I needed to cut two 30° slices from 24 bricks and I needed to get the sizes and angles just right, else risk wasting an expensive brick. Fortunately I found that by soaking the bricks in water, allowing most of that water to drain free, then cutting on an electric mitre saw, I was able to produce very nice cuts and virtually no dust - a real win win situation. You can see the outcome in this following video. But please, if you're going to do the same, don't use your best blade... else it won't be best for long.