[chasil]

Maybe try copper sulphate?

https://crystalverse.com/best-way-to-grow-copper-sulfate-cry...

Crystal growth has been on here before. Let me see if I can find a link or two...

https://news.ycombinator.com/item?id=31105320

https://news.ycombinator.com/item?id=30487511

https://news.ycombinator.com/item?id=29779923

https://news.ycombinator.com/item?id=29255511

[s0rce]

Sodium chloride will grow cubes but probably not trivial to get big ones.

[allan_s]

I did that with my kids and it was pretty cool !