During the course of the game, you can find up to 50 (sometimes missable) Red Jewels. By collecting them and giving to Gem the Jeweler, you can get additional rewards alongside the secret mansion in "Unlocking Gem's Secret Mansion" cheat.

Unlockable	How to unlock
Dark Power (Higher Dark Friar Damage)	Collect 30 Red Jewels.
Defense Force (Defense +1)	Collect 05 Red Jewels.
Herb	Collect 03 Red Jewels.
Life Force (Max HP +1)	Collect 08 Red Jewels.
Psycho Power (Higher Psycho Dash Damage)	Collect 20 Red Jewels.
Strength Force (Strength +1)	Collect 12 Red Jewels.