I wonder, the gem says unique, does that mean I can only have one in my gear? Or I can get another one after I socked first? Couldn't find answer really anywhere =(

Nope, just one anywhere on you, in your gear or inventory.

Unique means one on your gear, so if you later want get a new helm or whatever, you would have to dust or vendor the old helm with the unique gem in order to put the unique gem on your new gear. Annoying mechanic.

You can't loot another one until you DE/vendor/destroy the gear that contains the gem, as far as I remember.

They SHOULD change it to "Unique-equipped" as I think that is the intention of having such a restriction.

This is awfully specific for its own thread.

If you have one Unique gem anywhere on you, in your gear or elsewhere, you cannot loot another of the same type. Period.