shield materia

ok, i got this materia from northern crater (unless im mistaken...), maxed it, and brought the duplicate to level 2 so i could use it, assuming it was useful. only i never used it, until one of my attempts at emerald weapon. i cast it on cloud, and then used white wind. shield seemed to have stopped white wind from working (first time ive ever seen it miss/lose power), and seemed to do absolutely nothing against emerald weapon's attacks. so, what exactly does this materia do? how long does it last? is it worth the rediculous MP cost?

"Shield
LV 1: Shield
Absorbs all types of magical attacks, and makes you immune to
physical attack."

Maybe it doesn't work with emerald because his attacks are neither magic nor physical based, but based on your level.

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.

hmm. what about time duration though? about how long is it supposed to last? I can't remember if emerald attacked me before i tried casting white wind, so it could be possible that it simply wore off before emerald attacked me

As far as magical absorption goes, Shield is only effective against elemental attacks. Non-elemental magic will still have full effect.

I hardly ever use shield, but it is useful when fighting elemental monsters. I wish you got it earlier in the game tho.

I found Shield to be very useful.

Shield usually lasts around two, maybe three rounds (including the one it was used in).