They can simulate refraction and reflection.
Check out these beautiful eyes that look like jewels, made by X/Twitter user Omochi_Otoca for a VRChat avatar. The creator used a ray-marching shader, which can simulate refraction and internal reflection, according to one of their comments.
Once you purchase the avatar, called Beryl, the eyes can be found under Face Expressions in the game.
The model has other variations of the gem in different colors on her body. If you want a matching set of rings with the same effect, Omochi_Otoca has it in their profile as well.
Omochi_Otoca
To beautify your VRChat further, consider adding a pair of 3D wings or improving lip texture with this technique.
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