It is Makrooh (abominable) for women to wear rings made of anything besides gold and silver, e.g. rings of steel, copper, brass, etc. (Shaami)
*It has been narrated from Buraydah (radhiallaahu anhu) that the Prophet (sallallaahu alayhi wasallam) said to a man who had on him a ring of brass, " why is it that i sense the scent of idols from you?" The man threw away the brass ring. Thereafter he came with an iron ring on him, upon which He (sallallaahu alayhi wasallam) said, "Why is it that i see on you the adornment of the people of the fire?..." (Tirmizhi, Abu Dawood)
NOTE: Besides rings, it is permissible to wear other forms of jewellery that are not made of gold and silver.