I think something like a Bismuth crystal would have had the “arbitrary square” aspect they were going for, but would also have allowed that the cube might need to grow in any direction and might not always come out perfectly cubic at all times. I think this would have been somewhat ideal.

https://colemanquartz.com/cdn/shop/files/bismuth-midnight-blue-and-purple-crystal-mineral-specimen-488187.jpg?v=1727711991&width=1445