There are different tiers of ores in Terraria, each with its own set of ore types. The tiers determine the rarity and strength of the ores.

Ores generally appear as veins of placed blocks in the Underground layer and below, and are mined with a pickaxe or drill. Lower-tier ores appear among Dirt near the surface, or within Floating Islands. Some ores are also dropped by bosses, namely Eye of Cthulhu, Eater of Worlds, Brain of Cthulhu, Ocram, and Moon Lord.

There’s a total of 21 ores in the game. As you progress and improve your tools, you gain access to high-tier pieces. This is what makes the game balanced. As you venture into the deeper layers of the ground and encounter new enemies, you can take the nearby resources to craft new armor and weapons.