There are two types of Holy Paladins, those who spec and gear toward Flash of Light and those who spec and gear toward Holy Light.
Flash of Light Paladins are very efficient; their focus is on beefing up their Flashes and Holy Shock, and thus their gem of choice is the Runed Cardinal Ruby (+23 sp).
Holy Light Paladins are very inefficient; their focus is beefing their mana pool so that they can continuously cast Holy Light, and thus their gem of choice is the Brilliant King's Amber (+20 int).
Holy Light is considered (far) more viable than Flash of Light for ICC, and would thus be what I would recommend.