it depends on what type of gravel it is. if you are buying it it should say how much it weighs per square metre. and if its a school thing tell them this cannot be answered. sorry
Zircon has a average specific gravity of 4.65. (given by the density of the sand divided by the avg density of water) So, by re-arranging: the density is aprox 4,650 kg/m3 multiplying by g (avg) 4650...
One could argue that air has no weight, but it does have mass. At sea level and at 20 °C, dry air has a density of approximately 1.2 kg per cubic meter. Moisture actually makes it just a bit...