There is no rigid rule for ear-rings that I know of, but generally, Sikhs accept with gratitude the beautiful bodily form that God and given us and do not try to mutilate or modify it in any way, especially to look more charming.
Hope this helps,
MSB
Waheguru Ji Ka Khalsa, Waheguru Ji Ki Fateh! I do not know whether this is a correct argument but I was once told that Sikhs should not wear earrings because of its symbolism with slavery. I am sure that there are other more obvious reasons but apparently slaves would have an earring as some sort of identification. May I congratulate you on this fantastic service which I have only just come across. Many of my friends whom I have met at university have not been taught about Sikhi. They have many questions about why they are Sikhs and why that is important, and I am sure they will find this service most informative.