The explanation below is pretty clear. You get 1 diamond every time the number of silver gained from exchange has an extra 3 digits added to it. You can also see this when the unit name changes from K(1)->M(2)->B(3)->T(4)... and so on. Same goes when exchanging for gold, but you get 5 diamonds instead of 1. So K(5)->M(10)->B(15)->T(20)... and so on.