Rings made out of gold and silver are permitted.
Rose gold and white gold rings are also permitted if the gold content is predominant.
In principle, if the ring is made of an alloy of different metals then the ring will take the ruling of whichever metal is predominant in the alloy. For example, if the finger ring is made of an alloy containing 37.5% platinum, 10% silver and 52.5% gold, the ring will be considered to be of gold and therefore it will be permissible for a woman to wear it.
It is not permissible for women to wear imitation rings.
It is not permissible for women to wear platinum rings.
It is permissible to wear other jewellery besides rings for example, necklace, bracelet, etc. that are made from other metals besides gold or silver.
Source: The Ideal Woman