What can Life Crystals and Mana Crystals be used for after reaching max health and mana?

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.

Fallen Stars can still be used for ammo and other crafting material. Sadly, once you reach 400hp the only way you can raise your HP is by eating life fruit and life crystals become useless. The only reason for using them is to save them for a different charecter or to sell. Either way, fallen stars are still a good idea to collect but aren't as much of a priority than before you reach 200MP.

So to recollect:

• Life Crystals become useless and are only good for profit (Sell for 1 gold and 50 silver)

• Fallen Stars can be crafted into many different things along with mana crystals.

life crystals can also be crafted with chain to create a Heart Lantern! which restores health and stacks with the campfire.