Glass rendering incorrectly with dark areas I am a fresh newbie in Blender. I watched lots of tutorials from Blender Guru, and wanted to make a potion bottle. I don't know what went wrong, but the shading went completely wrong, and for some reason the main spehere doesn't reflect anything. If anyone knows why, please help me!

I used a glass material, mixed with diffuse for the bottle.
The dark areas are normals facing the wrong direction. Fix these by selecting everything A in edit mode and pressing Ctrl+N.  

To fix refraction issues, add a solidify modifier to the mesh. You might have to adjust the thicker part on the top. The glass shader is trying to guess where the glass is solid, so you should use it on closed meshes  only for the refraction to work correctly.  

For tinting glass, use a volume shader in the volume material output instead of mixing the surface with a diffuse shader.