Green sand huh ?
I guess you get coloured sand and fancy sand made from all kinds of rocks
I guess if you have a nice fresh river you`ll get some nice sand from it and rocks for sure, depends if you wanna clean them or use them to brew a tea if the river has good culture in it
Used to use a lot of cala sand or whatever it`s called when I kept reptiles, I`m guessing bugs must hate sand made from silica and volcanic rock but plants will love it
I just think a really fine sand would still hold a lot of water ?