Character eyes are black [duplicate]

I made a pair of eyes for a character of mine that consists of two meshes. One that acts as the real eye with the iris and the pupil, and then a copy of that one but slightly different to represent a cornea which has only a simple glass shader on. Why are the eyes of my character dark as soon as I place them inside the eye sockets? Seriously, you can barely see them. Can anyone tell me why Blender does this?

Eye out of socket:

Eye inside socket:

From a distance you can't see it's eyes at all, only if you're this close can you make them out.

• Try to increase the Transmission and Glossy bounces. – Denis Sep 7 '18 at 20:54
• It became a bit clearer but it's still too dark. I wanted them to be vibrant, almost like sapphires. – Pherion Sep 7 '18 at 21:12
• Can you attach the .blend please – WhatAMesh Sep 7 '18 at 22:03
• I added a dropbox link to the blend file on the post. – Pherion Sep 7 '18 at 22:32
• You probably need to pack the textures into the .blend, can't see them. EDIT: whoops, sorry, it seems only the World-HDR image was missing. – Manu Järvinen Sep 7 '18 at 22:34

I have played around a bit with your node setup, and what I've come up with is that from the Mix node that is connected to the Principled's Base Color, add a connection to a Bright/Contrast node, connect that to an Ambient Occlusion shader, and then an Add shader that adds together the Principled and Ambient outputs to the Material Output.
With the Bright/Contrast node you can control how bright/vibrant you want your eye, seeing as AO (and Add) doesn't have a slider for how much it affects the material. If you want a "strength slider" for the shader, or don't mind the eye emitting some light, you can exchange the AO shader for an Emission shader.