There are a few others I can think of that usually or always give space rocks if you make the right choices.
There's one where you're given a choice to land on an alien planet in the forest or ruins. If you choose ruins, and choose to take the treasure, you can get space rocks. If you choose the forest for this one, you can end up with either a live or dead alien.
Another one is where you're given the choice of whether to enter a race. You may need to have your rocket upgraded for speed for this one. If you choose to join the race, and choose to go all out on it rather than holding back, you will get a space rock as a prize.
A third one where you will earn a space rock is if you find a map leading to a planet marked with an X. Whether you choose to land on the planet or not, you will end up landing on it and finding a tree that you can enter. If you choose not to enter the tree, you will be given a space rock as you leave.
There is a fourth one where you end up with time reversed. Choose navigated a wreck when given the first choice. Make sure to remember all your choices and choose the same ones again once time gets set forward again. I've always chosen the option to warp subspace rather than do something with the hyperdrive, but I'm not sure whether that matters. The important choice to get the space rock here is that you should do a salvage mission rather than scan for signs of life.
Edit: I thought of a few more. There's a mission where an alien is tossing rocks at the sun and laughing. If you talk to him about it, he apologizes and gives you a space rock.
Another one is one where you think you are abducted by aliens, and they appear to be testing you. Choose the following: put ball in the round hole, wear the hat, sit next to the chair, and grab the jewels.
There's also a mission where you find astronauts throwing freeze dried ice cream or something at each other, and you figure they must have space madness. You should choose to confiscate weapons, and you may end up with a space rock.
I'll write more if I think of any others that reliably give space rocks.