Young modulus does not tell about the elasticity of substance >>>
But it states the stiffness which means rigid , inflexible and hard to bend .
of matter.

& you represented the values of stiffness of rubber and steel
such are

Rubber = .1 - .01 GPa
Steel = 200 GPa

so in your point of view diamond is more elastic than both of them ...

whose value is
Diamond = 1220 GPa