An iron or copper ring

It is not permissible for both men and women to use a ring that is made of iron or copper. 

However, if it is coated with silver, then the iron or copper ring will be permissible for man and woman both and if it is coated with gold, it will only be permissible for women. 

[English version of "Bague en ferraille ou en cuivre" – Al Jamiat – Vol.3 No.13]