And for anyone interested in the math that supports these empirical findings, for a number of explosions n, the expected gems exploded, g(n), is as follows:
g(n) = 36*(1-(55/64)n) + 24*(1-(29/32)n) + 4*(1-(15/16)n)
And Mana gain is 70% of that, or:
m(n) = 0.7 * g(n)