This is a PSA, reposting it from the WoW D&R, hopefully it gets hotfixed, likely will, but holding onto the Eye until then:
"Please be warned that at the end of your questline, you will be given an Eye of the Black Prince which applies your prismatic socket to a Sha-Touched weapon, but this eye is a ONE TIME USE ONLY! This means that once you use the eye, it is gone for good, and there does not seem to be a way of getting a second one!
This means that if you have an LFR or normal version of your Sha-Touched weapon, and you know you will be able to get a better version within the tier, I would HIGHLY suggest holding on to your Eye, and using it later. Don't make the same mistake I did.
Also Blizzard, you really need to add something to that quest that states it is a one-time use. People are going to be burning it on crappy versions of their weapons without knowing, and be stuck with that choice. This is especially misleading when the Sha-Touched gems ARE able to be bought again, and put into multiple weapons (just not equipped at the same time)."