Exploding a gem alone is 9 gems worth of mana, and while you’ll only get 70% of it for exploding gems, you’re still throwing 6 mana at your team with just ONE explosion.
The mana rounds to the nearest whole number, so anything <0.4 = 0; and anything ≥0.5 = 1.
EX: 1.4 mana equals 1 mana; 2.5 mana equals 3; etc.
Now take Azura who explodes ALL blue gems on the board, which could be zero (which is typically not an outcome unless there is a mana converter on the board who has recently turned all the blue gems), but you will average somewhere around 4 to 10 blue gems per cast, so you’re looking at somewhere around 36 to 90 gems exploded times the 70% modifier, and you get about 25 to 63 mana, maybe more/ maybe less.
That’s assuming that none of the explosions overlap one another - exploding two gems right next to each other would result in only 12 gems being exploded, 3 more than just one explosion would normally dissipate. But even considering that, gem explosions would help fill Azura right back up, so you’re almost always going to refill her per cast. And that’s not considering whether or not any Doomskulls were caught in any of those explosions!
Here is a really great post by @Lyya about the diminishing returns of exploding gems. It’s useful to gauge the effectiveness of Explosion type troops, as you can determine about how much mana you’ll acquire per cast of that troop’s spell.
PS: I’m an Explosion freak.