terraria
Comment from discussion You can't use expert mode items in normal mode..
Cenxx, a developer for Terraria, says here:
You guys voiced your concerns and we have listened. We will be making
some adjustments. There will however be 1 reward that remains
exclusive to expert, you will understand why when you see it. š
What is that one reward that remains exclusive to expert-only mode?
As Baked Potato said, Fallen Stars can be used to craft a number of objects - Most notably the Star Cannon, which can then use fallen stars as ammo for an incredibly powerful weapon. They can also craft a Space Gun which uses no ammo at all, made into Mana crystals to increase your mana up to 200, and can be combined with bottles to make a special lantern.
Life Crystals, on the other hand, as of 1.2 have no crafting recipes at all, and can only be used to increase your life up to 400 or for a quick buck. HOWEVER, they do make a very nice gift for your friends who are starting a new character!
Just...make sure they know that increasing their life can trigger events like Bloodmoon and Goblin Armies first. That...that can be a nasty surprise.
As of the latest patch, they can also be used to make Heart Lanterns.
Use the following link for the different ways to use glass.
http://terraria.gamepedia.com/Glass
The gamepdia for Terraria pretty much covers everything about crafting so if you have any questions of "how to craft __" or "what can I use __ for" I would check there first for the answer.
Best Answer
It's the Demon Heart. You get it when you open the treasure bag dropped from the Wall of Flesh (it only drops in Expert Mode of course). The reason you can't use it in Normal Mode is because upon consuming the Demon Heart, you get an extra, permanent accessory slot, which is probably too good to allow in Normal Mode.