Using your hoe in the entrance to the mines (where Carter and Flora are) to find jewelery and stuff can get you a ton of money, fast, because most items are 2,000G. But digging the entire dig site takes a lot of energy. So...
Save when you FIRST enter the mine.
When you find an item, remember where you found it, and reload your file.
Walk up to that spot, and dig.
Once you have the item, save again, and continue digging.
I made an easy 20,000G per day with that.