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The patch notes that changed this have your answer. I believe that we can only get purple at best. If I recall properly, gold jewelry only comes from the specific vendor in Cyrodiil and final bosses in vet trials for only trial gear.
Dungeon set quality is determined by the dungeon mode. Non vet will always be blue. Vet will always be purple. Treasure hunter basically raises the quality of the chest by one tier. Simple > Intermediate > Advanced > Master. With treasure hunter active you will always receive set loot from Intermediate or higher and simple will have set loot about 50% of the time.
This varies from overland chests which will have higher quality items in higher tier chests. Typically purple set items will drop only in Master chests, but with treasure hunter active, you will get purple set items from both Advanced and Master, making purple jewelry hunting much more viable from chest farming overland.
In both cases, it appears that gold quality items do not drop in chests. I've never seen one myself and haven't heard of any gold items dropping in chests.
Edited by redspecter23 on November 3, 2016 12:28PM
Since One Tamriel, I've opened a few hundred chests with treasure hunter. From what I can tell, what redspecter33 said is correct. More likely to get set items, more likely to get purples. Also works on treasure map chests (but not on dolmen chests).
I never got a gold.
It definitely makes chest grinding a lot better, but I think there is probably other grinds that are more profittable, so I wouldn't waste your green cp tree on this.