Refractive index=\frac{speed/of/.the /light/ in / vaccum }{speed..of ..medium}

this statement means that speed of light in diamond if   \frac{1}{2.42} times the speed of light in vaccum
The ratio of speeds of light, in a given pair of media , gives us a measure of the extent of refraction (or bending) of a ray of light as it goes from one medium to another. this ratio is called relative refractive index.
relative refractive index of medium 2 in respect to medium 1
= speed of light in medium1/ speed of light in medium 2
absolute refractive index of a medium(m)
=speed of light in vaccum/speed of light in the given medium
so here the absolute refractive index of diamond is 2.42