The ending of House Guest is quite difficult to interpret: Steven tries to repair the Geode with his healing saliva, and despite his regained confidence he fails. However there is no need to fear, as Greg quickly saves the day with a complex human tool: high-quality duct tape!
It shouldn’t be hard to understand why this explanation is a little hard to take at face value: the Geode is an enormous structure of hard stone which is containing an incredibly powerful force, and while that force is enough to widen the crack in the stone as Greg and Steven watch the duct tape is apparently enough to not just stop the growth of the crack but also contain the storm (which had been leaking into the surrounding area.)
The duct tape sparkled on Steven’s night stand, but then so did Greg’s van after Pearl fixed it—and she was giving a list of very mundane repairs she had done to it. That makes it a bit tough to be sure that the sparkles represented some kind of magical property being transferred to the duct tape, from Steven’s spit or from the sheer power of Greg’s belief (and what little hair he has left).
This mystery might never be solved. If it is, maybe we’ll learn where Steven’s healing powers went… But that’s a mystery for another week.