Something to do with trapping nighttime moisture.
I always thought because it was dark, it absorbed heat during the day rather than reflecting it.
This absorption causes a 'storage heater affect' thus making the beneficial heat available during the night.
The nighttime moisture you mention Ace could well have taken the form of condensation and give the benefit you describe.
In terms of coal/coke, and I am not so sure, as I think lava dust might be inert because it has been around for many years and weathered, whereas coal/coke might still be a bit toxic!
I am still out with the jury on this one!