These Bowl Gouges are made from a special steel called Vanadis 10; a high vanadium alloyed powder metallurgical tool steel offering a unique combination of excellent abrasive wear resistance along with good chipping resistance. This steel is known in the USA as A11 tool steel.
These tools have a parabola cutting profile which allows the wood shavings to flow easily and is prefered by most professional wood turners.
5/8" is the overall diameter of the tool not the flute size.