I think you're getting the Eviolite and Everstone confused.
The Eviolite boosts the Defense and Special Defense of a non-fully evolved Pokemon by 50%. There are a couple exceptions where Pokemon are NFEs (Not fully evolved) but don't get the boost, or Pokemon that do despite not being able to evolve (Male Combee and Salandit), more on that can be found here.
The Everstone prevents the holder from evolving, with some exceptions and differences throughout the generations, those of which can be found here. It also has an effect with passing down natures while breeding, which I'm not going to go into depth here, you can read about that on the Bulbapedia page.
The Everstone does not give any boosts to NFEs. The Eviolite does not prevent evolution. Both do not work on fully evolved Pokemon (in the context of evolution, the Everstone does work on fully evolved Pokemon when breeding).
Hope this helped.
