How many cubic meters to 1000 kg of boulders?

You cannot covert volume to mass.

Wet sand specific gravity is approximately 2.0 and dry sand is approximately 1.6Therefore 1 cu.m. of sand will weigh 1600 kg. if dry or 2000 kg. if wet.

1,000 liters = 1.000 cubic meter

there is one cubic meter that fits exactly in another cubic meter

You can't convert that - those are quite different things.