Hard Modifiers restrict how you're allowed to play rather than making individual blocks tougher — they remove or devalue an entire income source or system, then leave the rest of the run intact. A player can only enable one Hard Modifier at a time, as they are essentially distinct "challenge modes" layering on the existing game content.

Three of the six modifiers below target income directly (ONLYCOAL, COALISWORTHLESS, NOBONUSCASH), one removes the chest/passive system entirely (NOCHESTS), one removes a key shop upgrade path (NOTIMEUPGRADES), and one restricts which biome can spawn at all (ONLYCLAY).

Modifier Effect Strategy Note
NOCHESTS No chests spawn in the mine. Prevents all passive upgrades and equipment scroll rewards. Removes the passive system entirely. Very difficult without chest upgrades.
ONLYCOAL Only coal ore blocks spawn in the mine (all other ore types are replaced). All income comes from coal ($1/unit). Combine with mining speed passives from other sources if possible.
COALISWORTHLESS Coal ore sells for $0. Filling your coal quota earns you nothing, but you still have to do it.
NOBONUSCASH Bonus cash rewards (e.g. end-of-day bonuses) are disabled. All income must come from ore sales.
NOTIMEUPGRADES Time-based upgrades are disabled. You're stuck with the starting duration and cannot buy Add More Time upgrades.
ONLYCLAY Only clay-biome blocks spawn, regardless of depth. Mine is locked to the shallowest biome. HP/yield values remain at clay scale throughout.