Spell penetration gems

Bonuses for spell penetration can be found on blue, green, and purple gems, but the currently provides penetration as well. This is believed to have been an error in its itemization, but Blizzard has thus far made no mention of fixing it, which makes it a valuable gem for filling yellow sockets in PvP gear for DoT-based casters (who do not typically care about spell crit and may prefer spell penetration to intellect).