This page will show you the difficulty tiers given to levels and what they mean.
Friedelite: Free levels, non-demons/free easy demons
Pearl: Low-end easy demons
Simpsonite: Mid-end easy demons
Seashell: High-end easy demons
Cassiterite: Near-medium demons/free medium demons
Milarite: Mild/low-end medium demons
Emerald: Mid-end medium demons
Anorthite: High-end medium demons
Cymophane: Very high-end medium demons
Ametrine: Near-hard demons/free hard demons
Charlesite: Low-end hard demons
Charoite: Mid-end hard demons
Tourmaline: High-end hard demons
Wilkeite: Near-insane demons/free insane demons
Willemite: Low-end insane demons
Actinolite: Mid-end insane demons
Microcline: High-end insane demons
Microlite: Near-extremes/free extremes
Serendibite: Easy extremes
Hexagonite: Extremes considered harder than Bloodbath