Gravel or earth that contains valuable minerals is called pay dirt. If you're panning for gold in a rocky stream and suddenly realize your pan is full of gold, you've found pay dirt.
This North American term pay dirt dates from the 1950s, during the California Gold Rush. When miners found an area rich with gold ore, they would say they'd hit pay dirt. Today, you're more likely to use this term informally to mean "reward, profit, or success." If you win a fancy new TV in a raffle at your school, you might exclaim, "I totally hit pay dirt!"