Is it possible to just figure out the ratio of rocks and put them into a grinding machine to make custom sand?
Zombocom… the only limit is your imagination.
In all seriousness, rock crushing is energy intensive relative to just taking sand from the source.
You’re talking about a product that’s quite literally “pick it up and sell it”. Adding a single step to the process will be economically more expensive. Mining, logging, fishing etc. are all similar examples.
it would be better to farm coral reefs and parrot fish
Yes:
"Crushed rock sand – An economical and ecological alternative to natural sand to optimize concrete mix"
https://www.sciencedirect.com/science/article/pii/S221302091...
The main problem is that even though crushing rocks is affordable, illegal sand mining is even cheaper in regions where law enforcement is weak.