gah, too much to reply to!
Tell me about it...
So I could quite conceivably bury the surface with 1:1 returns, as my average fort runs 100-400 dwarves depending on just how high I want to set the population.
also you contradict your desire for the final major achievement in DF to become a mountainhall with hundreds of dwarves, as you could never accomplish this without flooding the map and the caverns with stone.
Im quite torn between whether true conservation of mass would be better, or should even be included, if it is, we need some efficient way to dispose of it in larger fortresses later on.
possibly something it could actually be used for that would save space? making concrete, powderizing it for various applications, some way to get it off the map basically beyond just stuffing it in a pile. however this should be a complex or expensive method, or you could possibly sell some of the rubble/gravel to humans, hell im sure they would love concrete.
Well, part of the solution can be as simple as off-map dumping. I doubt we could get automated carts all the way off map and back, but we could have the ability to literally just dump it somewhere out of place and out of mind/Memory by sending a dwarf off the map to landfill someplace a little further away.
Magma sea dumping or filling in eerie pits may be another ultimate long-term solution. (I kind of like the idea of dumping into the magma sea, but having a potential "eruption" take place if you do too much eventually happening, causing some magma to spit back out unless you can properly contain the flow.) (Eerie pits may just disgorge angry residents... The grues lost their home, and now they lurk the dark.)
Also, while this is probably beyond the tech range of dwarves, hypothetically, just as rubble/gravel is just smaller stones and rocks, sand is just smaller gravel.
Sand, unlike how it operates in DF, is not automatically glass-making material. Black sand, in fact, is an iron ore. White sand is made of limestone, and as such, is actually flux. Sand is generally used in glass production, however, as the quartz crystals (Aluminum silicate) that are most used in glass are the slowest things to erode down further into silt, and as such, are typically what is most left behind in typical sand.
If you built something specifically to tumble or erode down gravel further, you could hypothetically produce glass-making sand out of typical stones. In fact, the best stones to try this on are feldspars - I.E. microcline and orthoclase.
It's just... probably not technologically possible to do in any reasonable amount of time, unless you just stuck rocks directly under a waterfall and waited for erosion to take its course.
Sand, incidentally, erodes further into silt, then clay, then finally into kaoline, which is literally at the point where molecules are broken apart to make nothing but individual atoms. Loam is just a combination of sand, silt, and clay in roughly equal measure.
This is important to understand in farming, as the colloid grain size of the soil impacts the Cation Exchange Capacity, which in turn determines how much density of nutrients a type of soil can provide its crops.