Catalyst

Catalysts are currency items that add quality to jewellery. Quality on jewellery instead improves the effect of the implicit and explicit modifiers of the specified type, depending on the modifier tags. Modifier tags can be viewed by enabling Advanced Mod Descriptions in the UI options, and then holding the Alt key (default, it matches the item highlight keybind) while mousing over the item.

Using a different type of catalyst on an item that has quality will set the previous quality back.

Catalysts are acquired by defeating metamorphs in maps with organ parts that specify that they drop catalysts.

In terms of item filter, catalyst are classified as Stackable Currency[1].

Prior to Version 3.15.0, quality on jewellery also increased the odds of adding a mod of the matching type when using an Orb of Augmentation or Regal Orb or Exalted Orb or Elderslayer's Exalted Orb on the item, at the cost of reducing quality (2%, 5% and 20%, respectively). Such a behaviour is deprecated now - catalysts no longer affect the chances of rolling related modifiers on jewellery, and quality on jewellery is no longer removed when crafting.

Vendor recipe