What is a white socket and how does it work?
White sockets allow you to put any gem in it. The only current way to get a white socket is to use one of the Vaal currency on an item be warned though once you do this it is corrupted and you can not use any other currency on it. I'm sorry I don't remember exactly what it's called but it gives random effects.
Beside Descent of Champion races, which give you two white socket items, the Tabula Rasa is the only item in the game with white sockets.
You can put any color gem into a white socket.