Should be pretty easy to get by playing the game naturally but here's a couple tips:
1. Try to get gear with the Prospector Enchantment and upgrade it to max level, fully upgraded gives you a 300% extra chance to get Emeralds for killing an enemy and you could combine multiple gear items with Prospector for even more.
2. Look around the camp/overworld. Occasionally chests with 50 Emeralds pop up to claim.
3. Browse as much of each level as possible to find hidden chests.