All I can remember are two kids (at least one of them was a boy) finding a giant crystal in a cave. The cave led to a beach and they were chased out of it by someone and emerged from the cave, onto the beach at night. The crystal was very large and kind of glowed in the cave. It's driving me nuts!

The Goonies (1985) directed by Richard Donner

The Argon Quest (1990) - IMDb (http://www.imdb.com/title/tt0099055/?ref_=ttrel_rel_tt)