Lava's a Many Splendored Thing - S4.E4

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