I’d forgotten all about this book until I just read the obituary of the author Nicholas Fisk, who has passed away aged 93
A rain of tiny crystals falls on earth, seemingly harmless, but their odd shape and mobility means they can interact with each other to combine into new forms, and a local kid forms an unusual bond with their sort-of-hive-mind.... (at least that’s how I remember it)
Given it came out in 1971, I do wonder if my fondness for the “normal human fusing with shapeshifting alien” might have had it’s debut here..
Also recommended by the same authoer “Grinny” (After reading it ”You remember me!” as a phrase will linger in the saem way that “Are you my mummy?” does with Doctor Who fans)