This achievement is obtained by mining 200 ores/minerals underground. Mining any ore will count toward the progress of the achievement.
Once achieved, a player will be 25% of the way towards the Master Miner achievement!
Master Miner Achievement
The Master Miner achievement is obtained by mining 1000 ores/minerals.
Ores spawn in clusters and are relatively easy to find underground in any world. Equipping a flashlight accessory is useful in helping keep the underground well lit in order to discover these clusters. You don't need any fancy equipment, though: even the most basic pick will mine any ore.
Note that things like clay, resin, buried logs, and buried stones do not count as minerals.
This achievement originally required you to mine 250 ore/mineral blocks. At some point it was changed to read: "Mine 200 mineral blocks."