Farscape: Season 4, Episode 4

4 - 4: Lava's a Many Splendored Thing

While the crew has stopped off on a planet, Rygel leaves the rest of the group and is caught in an unbreakable orange substance as he's stealing treasures he discovers in a cave. When D'Argo, Crichton and Noranti go looking for Rygel, the cave seals them in. Two huge grunts appear who have protective armor which protects them from weapons fires.