Praise be to Allaah.
It is permissible for women to wear rings of gold or silver
with diamonds or other precious stones such as emeralds, rubies and
carnelians, or of iron if they wish, because the basic principle is that
they are permissible and there is no text to suggest that any of that is
not allowed.
It is not makrooh to wear a ring of iron. See the answer to
question number 105400.
Ibn Qudaamah (may Allah have mercy on him) said: It is
permissible for women to wear jewellery of gold, silver and gemstones
according to their customs and traditions, such as bangles, anklets,
earrings and rings, and what they wear on their faces and on their necks,
arms, feet, ears and so on. End quote from al-Mughni, 2/324.
It is stipulated that there be no extravagance involved in
that.
Al-Nawawi (may Allah have mercy on him) said: Our companions
said: All kinds of jewellery are permissible for women; it is only
permissible so long as there is no obvious extravagance involved. End quote
from al-Majmoo’, 5/523.
And Allah knows best.