Dirt

Dirt is a layer ore which you can mine from the start of the game.

Information about Dirt
Dirt spawns at surface to depth 250, where the next layer is Stone. To go to the Stone layer, you need to be Level 10, where you can mine to depth 750, where the Basalt layer starts.

This layer contains Copper, Tin, the first gemstones you will ever encounter (Ruby, Sapphire and Emerald, which spawn more common in the deeper part of the dirt layer) and many other ores. The only radioactive ores that spawn there are Hassium and Uranium. It's possible that other radioactive ores can spawn there, but they can be really hard to find. If you are at a high level, it's best to mine for ores in deeper layers instead, because those layers have more valuable ores.