Hi
I have a short suggestion, i hope my explanation will be clear. I think the marble piles' base deposit should be at least the double of its actual (but at least more). The amount 320 is depleted quite fast, and compared to the stone piles or to other mines depletion times its way less. I don't think anyone spending gems on marble deposits in shop, so this action wouldn't affect the "income".
I hope its a considerable suggestion.