It is impermissible for a male to adorn himself like a female. Rasulullah (Sallallahu Alaihi Wasallam) cursed a male who imitates a female and a female who imitates a male. (Bukhari)

The prohibition of imitation is general. A male should be distinct and not imitate a female in any respect. Therefore, males should not use any adornment attributed to females. Males are also prohibited from wearing gold, silk and other instruments of adornment. (Bukhari)

It is permissible for men to wear silver rings. The ring must not be more than 4,374g in weight.

In a Hadith narrated by Abdullah bin Bareera (R), who narrates from his father, that a person wearing a ring made of brass came to Rasulullah (Sallallahu Alaihi Wasallam). Rasulullah (Sallallahu Alaihi Wasallam) said to him, "I get the smell of idols from you.” This person flung the ring away and came again with a ring made of iron. This time Rasulullah (Sallallahu Alaihi Wasallam) said, "I see the jewellery of the people of hell on you." This person flung the ring away and asked, "O Rasulullah (Sallallahu Alaihi Wasallam) from what should I make the ring?" Rasulullah (Sallallahu Alaihi Wasallam) responded, "Make it from silver and do not allow its weight to exceed one mithqal." A mithqal is equivalent to approximately 4.374 grams.

From this Hadith the Jurists have deduced that it is not permissible to wear rings made of metals other than gold or silver. A man is only permitted to wear a silver ring weighing no more than 4.374 grams, whilst women are allowed to wear rings of gold and/or silver only.