I can't imagine how carbide would cut granite. I've attacked granite with carbide and the carbide just turned glowing red and then fell off. Granite is super hard, one of the hardest materials there is. You need to attack it with diamonds. The hardest part is drilling the hole. That is why I haven't made one, because I have lots of granite too. I have not found anyone with a drill bit long enough and the right size diameter that can drill it for me, and to buy a drill bit that is pen size, is extra money because it isn't a standard size. The drilling will require a ton of patience and lots of water. So if I had a 10mm diamond bit...do I want to sit there with the hose running over my drill press as I drill. Then after that, you have the turning portion. So now I'm soaking my lathe with water for ever and ever as I turn the pen. Turning would be done with a diamond file, or a diamond cutter mounted in a rotary tool and spinning slow. Tons of time and potential for damaged rusty tools. Once the task is completed, you have a beautiful heavy pen. Don't drop it, or all is gone...hard goes with brittle!
I've seen a few stone pens on the IAP. I think Gary Max maybe made them?