Nobody knows, but I am pretty sure that runes evolved from Roman letters when Germanic tribes carved letters into wood with knives (Romans used pen and ink). Runes have only vertical and diagonal lines; thus, they can be carved into wooden sticks so that problematic lines along wood grains or difficult circles can be avoided.
Basically, my rune-knife is a traditional wood-carving or leather-cutting knife, but it gets its name from the punched runes on it, and I use it for that very purpose. I am not sure how frequently Germanic tribes carved runes with this style of knife, but regardless, it is a proper tool for the job. Of course, you can carve runes with any knife or just scratch them with a sharp nail. The bare steel handle is traditionally improved by wrapping a leather strap around it, but on my personal rune-knife, I use a hockey tape ring between the edge and runes.
The rune knife is 80CrV2 steel that is hand-forged and hardened in a gas-fired blacksmith’s forge, and tempered in a kitchen oven at 170°C temperature, so it is very hard (about 63 HRC). I used freehand sharpening because it naturally produced a slightly convex profile, which is strong and cuts well. Approximate width 10 mm, thickness 2.5 mm, and length 120 mm.