Monday's Riddle! Lost then Found...

Short and sweet! Can you guess the theme this week? Good Luck! ~ F


I'm lost then found, dug up from the ground- something to save or hide away in a cave. I can re- write history or open up a new mystery. My value is set in your own clear mind. It all depends on what you find. 
What am I? 

Here's a clue: 
I am something that you really dig : ) FEAR NOT THE UNEXPECTED