Blademark Guild Adventures (Eberron)
The Augment Crystals
Augment crystals are small gems, crystals, or similar objects that imbue weapons, shields, suits of armor or other AC granting items (such as bracers of armor) with additional magical effects. They make magic item selection more flexible by allowing characters to change enhancements as they advance in level and face different challenges.
A +1 ghost touch bastard sword, for example, is a big commitment; a truedeath crystal (detailed below), however, is a small item that can be affixed to a weapon only when needed. If your campaign is undead heavy, the truedeath crystal might see a lot of use, but if encounters with undead wane in frequency, the truedeath crystal can be traded for a more appropriate augment crystal. This allows characters to add practical qualities to their favored weapons, without committing to items that could prove relatively useless over a large number of encounters.
An item can only hold a single augment crystal, but that crystal can be exchanged for another one at any time. Attaching or detaching an augment crystal from an item requires a move action, which does not provoke attacks of opportunity. In essence, each weapon contains an item slot that can house a single augment crystal.
Crystals come in three ranks: least, lesser, and greater. The rank describes two aspects of the crystal: its relative power level, and the minimum quality or enhancement bonus of the item required for the crystal to function. A least augment crystal functions when attached to an object of masterwork quality (even a nonmagical one). A lesser augment crystal functions when affixed to an object with a magical enhancement bonus of +1 or higher (such as a +1 longsword). A greater augment crystal functions when bonded with an item with at least a +3 enhancement bonus. Only the item’s actual bonus matters: a +3 dagger can house a greater augment crystal, but a +1 flaming keen rapier cannot.