Shield theoretically does what Gwel said - it nullifies attacks. Much like the invincibility items in FF8, this goes for any helpful magic you cast on yourself as well, which is why White Wind had no effect. Emerald Weapon is probably a special case, and I'm willing to bed that Shield has no effect on many or all of Emerald's attacks, either because of the way damage is calculated (i.e. the Air Tam Stomp dishes out 1,111 damage per materia equipped - though I can't think of what other attacks of his cause damage based on an equation other than the normal algorithms), or just because it's Emerald. Maybe someone has a better answer.