If you have 20 
quanta when you play it, does it block 20 damage total, even if you have gained another 20 quanta before it expires?
It blocks 20 damage total. When you gain (or lose!) water quanta, the amount of damage absorbed does not change, as the damage absorption is only based on when you played the shield.
Points to clarify/my current understanding of it:
Nacre shield blocks both physical and spell damage.
X is the amount of quanta you had after casting (like with stone skin), and does not change besides stacking.
Destroying Nacre shield gets rid of all the stacks.
Playing Nacre shield over PA'd Nacre shield does not remove PA?
It seems like you've combined Dissipation with SS. I like it! One problem I see is that dissipation field's multiplier is offset by spending the quanta to defend; because quanta isn't expended here, how do you justify the multiplier? The amount of defense you gain from 75 
is much less than the defense you gain from 75 if we compare the two. With 75 
, we are forced to spend all of the quanta we've stacked in order to defend(which can then be regained from quanta generation), in return for a multiplier of 3. I suppose you could say that SS is different because it stacks max HP, but since this does block spell damage, it's effectively a second health pool, and arguably superior to SS. Permanent control is an issue, but it doesn't seem like that's a big enough factor against it to justify such a big increase in power compared to SS.
From the top to bottom:
-Yes.
-Yes.
-Yes.
-Yes.With the multiplier, I guess I was thinking that compared to
,
quanta isn't as easy to generate unless you have a very specific mono-deck built around it. Sure, you can play an Enchant Artifact off the Earth mark onto the shield, but many decks nowadays have some form of stall breaking or rushing, which makes this card useful for more specific formats.
I guess the multiplier is a bit much, but either way, the mechanic is still decent in my opinion. Increasing the card's cost to say, 4, 5, or 6
quanta, or alternatively lowering the multiplier from 2 and 3 to just 1 & 2 are simple changes that can get the point across.
Side note - this shield is supposed to go hand-in-hand with Purify. You can basically block -any- type of non-Momentum damage, from Catapults, Unstable Gases and bolts, Fractal Psions, random creature rushes, Fahrenheit and weapons, bolts, and if you take poison damage? Cleanse it up.
