My glass(diamond) is reflecting nature, not transparent I am new to Blender 2.8.

I try to make a diamond, so I used glass BSDF.
But it shows me nature on the surface like metal, not showing the other side or some shiny refraction.
Its metallic is 0.

What should I change to look like a diamond?
You are using a glossy shader, which acts like metal.  I suggest using a glossy shader added to a refraction shader, it provides pretty good results.