An Extractor located in Port Hermalte.
The Extractor is a piece of technology from the age of the Ancients which can take finished materials such as weapons and armor and convert them back into the onyx (or altheum) used to enhance them.
Regardless, using an extractor costs money, and your output vary widely depending on several factors. The level, rarity, and amount of enhancement of the item play a factor. A fraction of the lustres used to enhance the item(s) are returned - typically 1 for each level of enhancement, with 0 to +2 extra per item. Extraction (after the Oct 2012 update) is net negative; you cannot profit from extraction.
Reset Sparks appear to extract crystal, onyx, or diamond Sparks.
Most major fields have an Extractor near the entrance. As of March 2014, they are also located near blacksmiths.