I have the gem eating chain, these are from the Q4 2016 GM Auction.
They add resists, crit padding, damage padding. The resistances are 40-60 percent which I think is random. They have both a timer and an amount of damage as for how long each gem lasts. I think the timer starts when you're first attacked/take damage. The gem will shatter when the timer runs out (think max is a couple of hours, counts down offline as well) or when you've taken enough damage. The gem quality determines timer length/damage capacity. Then you just insert another gem, I don't remember any rt from it.
Flare slot is open, script slot is taken up by the gem eating ability. Can't be padded since it creates padding, think temp resists can be added. Magic resistant so no player ensorcel, no player enchant.
As far as I know there's a resistance for every main damage type in the game, it varies by the type of gem. Here's a few that I know of:
Diamonds - VHCP
Uncut Ruby - VHDP
Black opal - impact resistance
Blue Moonstone - vaccum resistance
Fire Pearl/any pearl - puncture protection
a deep blue mermaid's tear sapphire - crush resistance
orange imperial topaz - fire resistance
emerald - slash resistance
rainbow quartz - lightning resistance
a chunk of pale blue ice stone - vacuum resistance