Resonances are special items that expand Item Gifting, allowing you to gift specific items to players you play with frequently, even if the item didn’t drop while you were playing with them. Resonances are specific to an individual player. For example, a Golden Resonance that resonates with Moxjet200 (can be seen in the tooltip of the Resonance) allows you to gift an item to Moxjet200.
There are two different types of Resonance that allow you to gift different varieties of items. Both types of Resonances are rare, but Golden Resonances are the more common of the two. They allow you to gift any normal, magic, rare, set, or unique item. Obsidian Resonance are much rarer, and allow you to gift exalted and Legendary items.

Slaying enemies that grant experience have a chance to grant progress towards a specific Resonance to drop, and once a certain amount of progress is gained, the Resonance will drop. This progress is cumulative between different play sessions and characters within a Game Mode, so when they share a stash, and only resets for a specific Resonance type when it is found.
Gifting an item with Resonance that has any restriction does not remove or change that restriction. For example, an item that requires to be aligned with Circle of Fortune to be equipped will still have that requirement after it was gifted.
Resonances are stored when you press the "Transfer Materials" button in your inventory, and can be found in your Gifting window. You can open your gifting window by pressing "Gifting" button in your inventory.
Frequently Asked Questions: Can I gain experience at max level (100)? - Yes. Can I gain experience at high level while playing in a low level area? For example, level 99 in a level 3 area. - Yes. You won’t get much experience, but slaying any enemy that grants experience can generate progress, and the amount doesn’t matter. Can I generate progress towards a Resonance just by being in a party with someone? - No. You need to be in the same area and within a relatively close proximity. The distance is similar to experience gain range. Can I track the progress towards a specific Resonance somehow? - No, the progress towards Resonance drops is hidden.