This is a very rare chance, but it will happen. If you mine long enough you can stumble upon a ? mark logo item. Its not garbage, but a piece of rock hiding something.(Rare of 5)
Usually it will be a weapon, it will be random, if you get a rusted sword upgrading it will make it the Ancient Sword, but at times you'll get common weapons like Iron Sword, so you have to get lucky.
Place that I found it is in the Volcano area(once you hit HR 18 you'll be able to explore online there)