The problem with quantum cap modifiers is that they have to be really really huge in order to have even a slight game effect. Your design here, while (as far as I know) it is unique, does not modify the cap in any game-impacting way at all. Even having six of these out would be essentially meaningless in terms of the quanta cap.
Having said that, though, I'd actually still use this card just because of the +1 quanta/stack ability. In decks using both pillars and pends of a particular element, it would be producing two quanta/turn, and it would pay for itself in a few turns over a pillar/pend. It would also open up the rather interesting strategy of deliberately using unupped pillars/pends in an upped deck so that you could have three or even four stacks of water producing cards. (or whatever other element) Heck, with unupped/upped marks, you could theoretically get this as high as 5-6 quanta/turn produced by one Glowing Crystals under the correct conditions.
Zanz had thrown out an older idea for the aether shard which would halve the opponent's quanta cap (cumulatively) with certain minimums (exact #s I forget now, but they were lower if your mark was aether, since it was the aether shard) That's about the kind of scale you'd have to implement for it to mean anything. Something like a +/- 10 cap modifier, perhaps? That way, six of them would reduce a quanta cap to 15, so all cards would still be playable, but it would still be an inconvenience to most decks. The problem with that, though, is that having 5 out (and thus a 25 quanta cap) would be meaningless to 90% of all decks, and so unless you drew them ALL, it wouldn't do anything.
Another possibility would be to have the cap modifier progress geometrically. For instance, the first crystal is -3 quanta, the next is -6, the next is -9, the next -12, then -15, and -18. (total -63) You could also do it the other way around, having the first one be -18, and the sixth -3. Both have advantages and disadvantages. The first possibility means you'd still basically need all six to have any meaningful effect, and it also means that dexterous arena decks could completely lock you out with 7 or more of them. The second possibility fixes that problem (more than 6 would do nothing, like having >4 shards of freedom does nothing now) but it also weights the first few crystals quite heavily. This isn't so much of a problem, though, because since the cap starts so high at 75 already, you need the first ones to have a big number value to have any impact at all. I think this progression is actually fairly balanced, so I'll make a handy table:
1 glowing crystal out = opponent's quanta cap is 57 (75-18)
2 crystals out = opp's cap is 42 (75-18-15)
3 crystals out = opp's cap is 30 (75-18-15-12)
4 crystals out = opp's cap is 21 (75-18-15-12-9)
5 crystals out = opp's cap is 15 (75-18-15-12-9-6)
6 crystals out = opp's cap is 12 (75-18-15-12-9-6-3)
Note that even though the 6th crystal is only a -3 modifier, because you're going from 15 to 12 quanta, this is actually quite a huge difference. It completely locks out Miracle, for instance, and makes it so several other cards like dragons will drain an entire quanta pool of that element.
This also doesn't touch the other side of your crystal idea, which is increasing your own quanta cap. Having played in the old Elements where quanta pools were uncapped (I've seen pools with over 1400 quanta :p) I can safely say that the only cards that there would be a practical issue with are Stone/Granite Skin and Dissipation Shield/Field. For stoneskin, all that would have to be done is enforce the 75 cap on the card itself that used to exist before the quanta cap, or possibly even just ignore it and let the 500hp cap take care of huge stoneskins. (the disadvantage there is one stoneskin can potentially give you hundreds of hp then, rather than needing all six to get that high) And as for dissipation shield/field, let's just say that it kinda sucks when you're up against someone with thousands of quanta :p Bolts and Fahrenheit, despite the whining of a few players, didn't really have that much of a problem against skilled players. Even then, using all six crystals to get a 138 quanta cap seems reasonable to me; that's six cards you'd have to get, and that starts to not leave a lot of room for cards like bolts or whatever..
