If you put a diamond in a crafting table with the crack hammer, you get two diamond powder which you can then smelt to make two diamonds. You can do this endlessly as long as you have a fuel source and the ingredients for more crack hammers. Don't know if this is intended but if so then it's extremely exploitable. An easy fix would be to change the recipe to either only make one powder for diamonds and emeralds or switch the ingredient to diamond/emerald ore.

No, that was not intended. It was a "stupid cut-n-paste" mistake on my part, forgetting to change the quantity to 1. Thank you for reporting and I will update quickly.

Fixed in v58