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

True, but among the minerals with cubic crystal structure it is not unusual for them to be found as crystals that are perfect regular or semiregular polyhedra, with a shape characteristic for the mineral, for instance octahedron (e.g. spinel, diamond), rhombic dodecahedron (e.g. garnet) or cube (e.g. pyrite).

I suppose that the crystals from the picture are of pyrite, which frequently looks like this.

In the antiquity, when what are now called diamonds (the Romans and the Greeks called them "Indian adamants", because they were first encountered by Europeans during the expedition in India of Alexander the Great; "adamant" meant something else in Europe) were very difficult to cut and polish, they were normally used as gems in their natural shape of regular octahedra.

Cutting diamonds from their natural octahedral shape into polyhedra with more facets, e.g. brilliant, was invented much later.

Pyrite or fool's gold, lovely mathematical perfection and a great etymology to match!

Especially since it's an exception that breaks the rule that straight lines are not found in nature. Not only is it a straight line, but a cube. They just look unnatural. Very cool stuff

You can buy pyrite cubes on Etsy—I know because I also love them :)

They’re not expensive

That pyramid shape in the amethyst is what grabbed me. Looks like something straight out of a video game. Incredible.

What about organic mishmashes that are shaped into cubes?

https://www.science.org/content/article/how-do-wombats-poop-...