For those who can't sleep for worrying about what's going to turn up in the green gloop, here's a quick note to let you know. After the first day, long clear needles started appearing on the string - up to 2 cm long. As soon as I got the camera out, though, they'd all vanished!
This does happen, sometimes - things crystallise, and then dissolve again. This time, though, it was more mundane - the crystals had got too big and fallen off!
So, a quick decant later, and here they are, growing together in the bottom of the beaker:
The green is just colouring from the solution itself, and don't worry about the brown blob on the right - that's the remains of a paperclip I dropped in for fun. As I might have said, it's good stuff, this!
Now, I know you're dying to know what this is, so here you go:
The crystals have a parallelogram-shaped end that slopes to one side, but are otherwise long thin prisms. Of the possibilities from what went into the gunk (sodium chloride, copper sulphate, and a bit of aluminium) this can only be...