Stone Mask Achievement in Vampire Survivors

The first place you'll be able to find and pick up a Stone Mask is on Stage 2. One always spawns on the map on the right side of the level, at the top of the screen. Once you acquire the Milky Way Map relic on Stage 2, you'll get a map when you pause the game. Do so on Stage 2 and you'll be able to see the Stone Mask on the right that you can then track down and pick up.