The amount of bonus that a Gem grants depends on the level of an item. In a level 36 item a normally bonded Gem grants a +3 stat bonus, while a superior bonded Gem grants a +6 stat bonus.

When a Gem finishes imbuing into a socket, it has a chance to form a superior bond with the item for an additional bonus! Once you have a Gem socketed into an item, you can replace that gem with a new one and have a chance for that replacement to yield a superior bonded Gem! Replacing an existing socketed Gem takes 12 hours, regardless if it is in the 1st, 2nd or 3rd Gem slot.
what is superior bonded gem?