Health Crystal
To craft the Health Crystal you need 2 Life Crystals.

This item cannot be used until you reach Level 51 after defeating the Wall of Flesh and unlocking Hardmode.

At level 50 and below, you have a maximum of 400 health. Upon reaching level 51, using this item will give you 20 health added to your max until you reach a maximum of 800 Health.

This item doesn't work in N-Terraria 4, but can still be crafted from 2 Life Crystals, and can be crafted back at any given time.