An augment crystal 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, Least, Lesser and Greater. You must meet the level of a crystal to be able to use it.