Moon Stones are used to evolve certain Pokémon, but they are not available in Pokémon Crystal.
Here are some of the Pokémon that evolve using a Moon Stone in other games, but not Crystal:
* Nidorina -> Nidoqueen
* Nidorino -> Nidoking
* Snorlax -> (No Evolution)
If you're looking for a way to evolve these Pokémon in Crystal, you'll need to find another method or trade with someone who has a Moon Stone from another game.