Which gem boosts the quality of amulets or rings?

I've only had the game for a few weeks, but I've not yet found a way to do this with them, although clearly they can have a quality above zero.

That would be an Orb, and I do not believe there is an orb that increases accessory quality.

There is none, because these items do not have a base stat (eg armour, damage).

However, you can roll better rings and amulets by using your currency items on items with higher item levels. To see an item's level, pick up the item by left clicking it and type /itemlevel in chat.