I've just found following picture, but I don't know how to recreate this kind of crystal type reflection thing. Can anyone teach me please?
As far as I can see in the picture and the material node provided from creator (which contain some errors in my opinion) the reflection is achieved by simply mixing a glossy shader and a diffuse shader.
The sharpness of the reflection is driven by the "Roughness" value of the shader:
High Roughness value
Low Roughness value
First of all, you need to choose a reflection image. (I love using city landscapes off of http://cgtextures.com/.
Then add a new texture and set the mapping to "reflection".
The image is calculated in realtime, so its 100% game functional (and looks beautiful)