First of all you must have a trade portal max level or have lots of gold ingots and sticks.
Craft a gold spade.
Eventually after repeating this you will be receiving black sand.
Sell the bonus items you received.
This can be done with dirt too but the bonus is not as good.
Thank you @Blocknuggets for letting me know about this part below
Mining stone that is not placed while using a gold spade is better than using a gold pickaxe and you will receive major profit.