In Final Fantasy XII, when you libra something, there are little icons that say what they are weakened by....?

red flame - enemy is weak against fire,

light blue ice crystal - enemy is weak against ice,

blue drop - enemy is weak against water,

yellow lightning bolt - enemy is weak against lightning,

green leaf - enemy is weak against aero (wind),

white sphere - enemy is weak against holy,

black sphere - enemy is weak against dark,

brown earth - enemy is weak against earth

Weapons with holy element - Excalibur, Wyrmhero Blade, Holy Rod, and Holy Lance. Also, Holy Motes cast Holy.

Here is a good weapons guide:

http://www.ff12sector.com/ff12_weapons.php

They're elemental if I remember right. I think the color of the "rock" tells you what its weak to. For instance, Red would be Fire. Brown would be ground/earth. Blue would be Water. White would be Holy. Black would be Dark. Yellow would be Lightning. etc.