The Gate (1987)
There's a Passageway - A Gate Behind Which the Demons Wait to Take Back What Was Once Theirs.
Two teenagers come across a special rock with a beautiful crystalline center in their backyard. The rock enthralls them and they dig up the family's lawn in a search for more of the precious stones, but instead find the Gate. It's a hidden underground chamber that holds the secret of centuries and the vengeance of eternity.