Let's begin by understanding the key concepts and definition of density: 

Density is a characteristic property of a substance. Therefore density is de fined as the relationship between the mass of a substance and it's volume i.e. the space it occupies or takes up.  The arrangement, size and mass of atoms determine the density of any substance. 

NB: Substances or objects with the same volume but a different mass will not have the same density, also objects with the same volume but different mass will have the same mass.

Density can be written mathematically as follows: 

`D = m/v`

Where D = density , m = mass and v= volume

To answer your question about the density of your rock we use the above equation: 

`D = m/v`

`m = 50g`

`v = 10ml`

`D = (50g)/10ml = (5g)/l`

SUMMARY: Density, D, is 5g/l