The Gate (1987)

With their parents away for the weekend, teenagers Glen (Stephen Dorff) and Al (Christa Denton) get excited by the discovery of a strange crystalline rock in their backyard and elect to start digging into the ground around it. However, the pair soon find they have found the gateway to Hell and have unwittingly unleashed a terrible force into the world. Will they be able to seal the portal before the Earth is overrun with evil?