Hamlet of Sheffield's 1/2 high speed steel bowl gouge made from a 5/8" diameter rod.   The 1/2" designation is the English measurement system of gouges--in North America this would be a 5/8" gouge. 
This is the standard bowl gouge in almost all English-speaking countries in the world.  Versatile enough to do bowls from 5" to 30" in diameter.  Made from M2 high speed steel, it has excellent edge-holding qualities and is not damaged by bluing from grinding.