I'm trying to remember a YA book (could be a short story, but I feel like it was a novel). I read it a long time ago (approximately 30 years) in the US, in English--do not believe it was a British novel. I have no memory of the cover or author.
Setting: it was a colony world setting. Young girl as the main protagonist. The secondary protagonist was an older woman. I feel like she was a scout type that was on the planet longer than the colonist and ostracized by them (similar to Keeper/Guardian of Isis, but not that series). One major characteristic of the world are these large crystals present everywhere.
Plot: at some point, everyone in the colony is stricken with a "disease". I want to say that are all held immobile and unresponsive. The only ones moving are the girl and woman. They investigate and later determine that both had actually consumed some of the crystals ("looked good enough to eat") and were thus inoculated from the symptoms. They are then able to bring back all the colonists.
I know there are some similarities to the Green Book by Walsh, but it is not that. Not McCaffrey's Crystal Singer books, either. I don't believe it was a series at all.