Today i found out how to effectively sell a stack of coal for 500 Nz. All you do is purchase 8 stacks of sand, smelt it, and sell the glass. The sand should run you about 1500, and the glass should sell for around 2000, and all you have lost is a stack of coal, therefore 1 stack of coal =500. therefore by the same logic, 2 stacks of coal= 1 diamond, if you buy it from the store. With coal being the most abundant ore in the game, and with it being possible to get a whole stack from just one vein, and with coal not being sellable in the game, this is a very handy glitch. It's not even a glitch, its like a loophole. Roughly 8Nz per coal. Im not sure if this is already done en masse here, but i just discovered it, and made 3000 from a few stacks of coal people gave me for free.