Pink Ballast
About 60 years ago the C&NW Ry bought a mountain of Pink Quartzite in South Dakota or one of those other nearby states, it has been used ever since as ballast on the now Union Pacific former C&NW tracks.
Here is a statement from Wikipedia:
"Quartzite is a very hard metamorphic rock that originated as sandstone. Through a process of high heating and pressurization sandstone is transformed into Quartzite, an extremely strong and durable natural stone."
We live close to the UP line that goes toward Harvard, Illinois, so for our railroad we decided to use that pink ballast, it is much much smaller than the original.
Because this material at our size is costly, and does not really have much cohesiveness, we only use it as a top coat, underneath is CA-6 Grade 9 road gravel which is much cheaper, can be compacted and stays in place.
Below are two photos, delivery, and use as a top coat: