Question

Is there something you are supossed to do with the ice stone in twist mountain?

Answer 1

The icy rock is the method for evolving Eevee into Glaceon. You bring an Eevee, level it up around the icy rock, and it will evolve.
Source: Experience.

Answer 2

It evolves Eevee into Glaceon when you level it up near the rock.

Source: Experience

Answer 3

Eevee, when leveled up in the presence of the Icy Rock, evolves into Glaceon. That's the only purpose the rock serves.

Comments

My source is also experience