Acquired by completing a romance plot in the Shuttered Palace; while you possess a Memento the romance plots are closed to you. The Memento can be auctioned off (access by clicking on it) with a Scandal challenge (straightforward at 11; note that you can equip items and then click on the Memento without being banished to the Tomb-colonies; just be careful to click on the 'ME' screen rather than hitting 'Done' and unequip the items before returning to the story screen):
Sell your Memento of Passion at auction
Did it mean nothing to you? Is the memory too painful? Or are you just very short of ready cash? In any case, once you've sold it, you'll be free to attempt other avenues at Court.
'Lot 111. A token kept by a certain gentleman of… complicated reputation, as a memento of an affair of the heart with one we will not name here. Ahem. Do I hear one Echo…?'
scandal challenge, straitforward at 11 (!)