First, make sure you have a Supreme Pickaxe. For 3,000, it's a pretty good investment as long as you always check the durability of it. Next, follow these steps:
1) Exit Town of Amarance and go to the map left of it.
2) There will be one Mountain Murdoc on that platform and a mining rock. Kill the Mountain Murdoc for some exp and to get it out of your way.
3) Mine the rock, and get the items from it.
4) Return to town, rinse and repeat. If the rock does not appear when you return to the map, return to the town and go back. The rock should be there again.
From killing the Mountain Murdoc, you may get some good equipment and gold, along with some Deluxe Spice Sets, which sell for 400ea. From the rock, you will get mostly Thorium(which sells for 250ea. It may not seem like much, but you get a lot of Thorium), along with some other stuff like Emeralds(which sell for 300ea), Sapphires(which sell for 400ea), and Diamonds(which sell for 200ea).
Keep doing this until your axe goes down to 10 durability, and then go to the Amarance shop, sell the ingredients, and repair your axe. If you have followed everything correctly, you should be getting about 10,000 or (probably) more from this method.