Unless of course you catch one or more of the gem carrying scouts following around the defeated army. I once uncovered 5 such scouts (packing 30 gems each) after a particularly massive battle. My opponents loss of 150 gems brought a smile to my face. Imagine if I could have recovered all those gems for my own.
or perhaps when gems fall on the ground they materialized into the the earth, air, or whatever, and the number of gems that are lost by a carrier adds to percentage of a random event in which to gain gems