Bro and sis search for crystal and long-lost father
I'm trying to get the title of a children's movie from the 80s/early 90s. A brother and sister are accompanied by this guy to look for a crystal rock and to find their father (I think he went off to war). They travel by boat, and there's this part where they go to a monastery and find skeletons of monks who were making books. There's also a part where they are going into a cave filled w/ dogs or creatures that are guarding the crystal. I think the guy w/ them turns out to be their dad.
I always thought the name of this movie was The Dark Crystal, but alas, it is not. Whenever I try to search for it I get the Jim Henson movie...
Please help!