"Infinity" key

[madbananas]

I've found this locket of amber with a hair with the infinity shape inside of it, but besides that I can't see I'm missing any room to explore where the "infinity key" may lie.
I suspect I need to combine it with some other item in order to get the key, but I have no clue at all about what could it be.
I'd greatly appreciate if anybody that already got it can throw some clues this way ^_^

[superpamperus]

I seem to recall that there is a ouroboros shaped key, but there is no infinity key (far as i know)

regarding the amber locket

the amber locket can be used on Magda's portrait, in the earl's bedroom

[madbananas]

Graciasssss!