dust can remain airborne for several minutes, even hours, yet it has the same density as the rock that spawned it.
Yes, and ..no.
Each tiny little speck of rock dust is the same as the rock it came off of, BUT density is a concentration per unit volume. A cubic centimeter of rock and the same volume of rock dust in air are not the same density, despite each particle of dust being the same as the rock. IT is a mixure (dust particles and air), and has a different makeup, and density than the solid rock.
The pull of gravity is constant, (at a given distance from the center of the earth), so the pull on objects is the same. All try to fall at the same speed, because they are being pulled down by gravity at the same amount.
It is the relationship between surface area of the object, its weight and the resistance of the air it must pass through that changes the velocity different objects attain as they fall. This is where "terminal velocity" comes in.
Drop a 200lb man and a 200lb lead brick from high enough, and each, as they fall through the air, will eventually reach different terminal velocities. Both are under the same "pull", the acceleration due to gravity, but the air resistance to the falling man's greater surface area will eventually reach a balance with the pull of gravity, and the body will not fall any faster, no matter how much further it falls.
The same thing happens to the lead brick, but because the surface area is many times smaller than the man, the brick will accelerate longer before air resistance balances the acceleration due to gravity. Much longer, actually. SO the brick is moving much faster by the time it reaches its terminal velocity.
It may just be semantics, or your point of view. Mine is that everything falls at the same rate of acceleration speed (the pull of gravity), but it is a combination of weight, surface area, air resistance, and time that determines the final velocity a falling object reaches (on earth).