What is the difference between 2a modified crushed stone and minus gravel? I understand that both are crushed stones and both have finer particles. Do they come from different species of stones? What are the differences in properties and applications? My research seems to indicate that the two terms are used interchangeably, but I suspect that there is a difference.
Generally anything that is "modified" means it has some limestone dust mixed with it, the idea is that once it is compacted, over time as rainwater falls on it, the water works down through it and causes it to harden into a unified piece of rock like concrete. Sometimes they call it "treated" instead of "modified" The "minus" stuff is just whatever comes out of the crusher. For example 3/4 minus is gravel that has rocks no larger than 3/4" but has rocks that can go down to the size of sand grains.
As for what goes into it, "minus" can be anything from broken chunks of concrete, to actual rocks, to a mix of bricks, rocks and concrete all fed into a crusher and sifted to keep rocks out of it that are larger than the rating.
The minus stuff is generally what you put down when you are planning on putting asphalt or concrete on top of it.
The modified is what you put down when you aren't going to cover it but want a surface that will eventually firm up so you can park cars on it, etc. It can take a year for that to happen, though.