An io rune is a low tier rune that can be socketed into armor and weapons. You can cube 3 hel runes and a chipped diamond together to get an io rune. You can also obtain an io rune from nightmare countess. You can get an io rune from the nightmare hellforge quest.
An io rune requires level 35 to use.
Horadric cube recipes with io runes
transmute 3 io runes and a flawed topaz to create a lum rune