Onyx

Black onyx is believed to be one of the most powerful protection stones. Its way of doing this is to absorb and transform negative energy. Because of this quality people find it helpful in relieving stress and for emotional stability.

It has been said that in India it was commonly worn to cool the ardors of love. Thus it may be very helpful for people who can't let go of past relationships (sometimes even long after they're over).  The same can be true of any belief which no longer serves one - and they can be as hard to let go of as past relationships.