I read this novel or novelisation one afternoon at the city library somewhere between late 1984 and early 1986.
It was set in the Star Wars universe and the main story was that there was some sort of crystal on a planet that could amplify /boost the power of a force-user/adept.
Luke and Leia are both on the planet where this crystal is and are trying to stop the Empire, specifically Darth Vader, getting hold of it.
I think they find a small splintered-off piece of the crystal early on and find that it works as advertised and are then terrified of what the whole thing would be able to do.
At one point Luke is incapacitated or they get separated (not sure which) and Leia faces Vader alone. She has this total BAMF moment where she fights Vader with a lightsaber and drops him off a cliff.
I don't remember the ending or much else of it, although I think the crystal was either destroyed or buried beyond any force user's ability to retrieve.
Sorry I don't have more details, but I was only 13 or 14, and speed read it in under two hours while waiting for a friend to finish at the dentist, and it seems like another lifetime.
Now that we know the answer, here are some other key events and terms to help other people who may be looking for the same thing and that was not in my original question (taken from Wikipedia, Del Rey website, quotes from the ebook, etc.)
- Luke doesn't know Leia is his sister and story has some romantic undertones between them
- Luke and Leia are on their way to a meeting on Circarpous IV when they are forced to land on Mimban (Circarpous V)
- Luke slaps Leia and calls her a servant girl to avert suspicion
- Luke falls into the mud, and pulls Leia down with him
- Luke and Leia wrestle / play-fight in the mud
- Luke and Leia hear about the Kaiburr crystal from an old woman called Halla and are shown a splinter of it
- Luke and Leia find an abandoned, underground city, where they are stalked and attacked by Coway natives.
- To save his friends, Luke fights and defeats the Coway's champion fighter.
- After the Imperials are pushed back by the Coway Darth Vader kills Captain-Supervisor Grammel for the defeat
- Luke's leg is pinned under rubble
- Leia takes Luke's lightsaber and fights Darth Vader.
- Luke severs Darth Vader's arm.
- Vader falls into a pit
- Luke and Leia's injuries are healed by the crystal
"But what if Master Luke is correct and there is no station below? We could find ourselves marooned forever on this empty world, without companionship, without knowledge tapes, without … without lubricants!”