An augment gem is a small gem, crystal, or similar object that provides a magical effect when affixed to an item. Each item can hold a single augment crystal, but an attached crystal can be swapped for another one at any time. Attaching an augment crystal to (or removing it from) an item requires a move action that doesn’t provoke attacks of opportunity. Effectively, each eligible item has a single “slot” that can be filled by any appropriate augment crystal.
Crystals can come in 3 forms, Lesser, Moderate and Greater. You must meet the level of a crystal to be able to use it.
Lesser Gems – 3rd Level
Moderate Gems – 9th Level
Greater Gems – 15th Level
You get the effect listed for your rarity of gem plus the effects for the lower rarity gems. (ie. a greater gem gets the moderate and lesser effect) unless stated, HOWEVER bonuses of the same kind do not stack, so you cannot wear two gems of warding to get a higher DR, but you can wear two gems of elemental defense provided they are for different elements.
NOTE – 2 indentical gems can be fused to create a gem of the rarity up. (ie two lessers make a moderate.)