1400 years ago people thought that all stones sink in water. However today we know that there is an exception. One kind of stone actually floats on water.
Density is simply mass per volume. Density of water is 1 g/cm3, density of gold is 19.3 g/cm3... while Earth has an average density of 5.51 g/cm3. But to float on water you need densities lower than that of water.
These are densities of rocks and soils as found in nature:
Clay has a very low density of just 1.2 g/cm3 but still higher than the density of water, which means it would still sink in water. However if this clay gets heat treated, in a volcano for example, it would become porous and its density drops.
The density of this pumice stone is so low at 0.25 cm3. This stone literally floats on water.
Pumice floats n water.
Pumice stone is the only stone that can float on water.
Today we know that pumice stone are the least dense stone on Earth, however 1400 years ago this was portrayed in the Quran:
Pumice "Sijjeel in Arabic سِجِّيلٍ" is also found in Quran 15:74 where the city was showered by a volcano. Today we know that pumice is very porous and has very low density which means it is very easy for the birds to carry. If those stones were made of gold, for example, then they would have weighed 77 times more and those birds wouldn't have been able to fly.
How could an illiterate man who lived 1400 years ago have known which is the least dense stone on Earth?
The Quran (Koran, the book of Islam) contains scientific knowledge that could not have been known 1400 years ago. It ranges from basic arithmetics to the most advanced topics in astrophysics. You are invited to go through those miracles and judge for yourself.