The Mining spot will unlock as part of the Animal Safari when Agate arrives around Fall- Year One. Make sure to forage here every day!
The mine will give you some awesome items to sell and to use in crafting.
The items that you get out of the mine will depend on the quality of your hammer. An Old Hammer will yield mostly rocks and glass stones, while a better quality hammer will yield more pricey minerals, like Emeralds and Rubies, as well as pricier metals like silver, gold and platinum.
The Master Hammer will be your best best to get more platinum, gold, adamantite, sand rose, and orichalcum.
Make sure you bring a snack and have some space in your bag. Spaces fill up quickly when you find only one of several items.
Any mineral (except Pearls) can be mined randomly from the Mining Spot.
If you're getting junk wait 7 days and the quality will go back up.
The Philosopher's Hammer is your best bet for Platinum, Gold, Adamantite, etc. not the Master Hammer which is one level down.