Some of the levels are marked hard level by king.com. Most of them are rather difficult. There are currently (including Episode 114) 405 hard levels (20.88% of all levels) in Diamond Digger Saga.
In the Episode picture the icons for hard levels are marked with a red circle 
What makes a hard level[]
- High target scores (Score levels only)
- Few moves
- High number of different gem colors
- Magic Orbs or chameleons or color locks requiring many gems
- Lava bombs that explode after few moves
- Narrow channels
- Impossibility to miss a single toy or firefly
List of hard levels[]
- Level 25 - The first hard level.
- Level 130 - The tenth hard level.
- Level 430 - The 50th hard level.
- Level 646 - The 100th hard level.
- Level 845 - The 150th hard level.
- Level 1018 - The 200th hard level.
- Level 1183 - The 250th hard level.
- Level 1445 - The 300th hard level.
- Level 1658 - The 350th hard level.
- Level 1903 - The 400th hard level.
A (currently incomplete) list of all hard levels can be found here