Short answer
Women may wear all kinds of jewellery, including gold, silver and gems. Men are limited: in the Hanafi school a man may wear only one silver ring, whose stone can be a diamond or any gem. Chains, necklaces, bracelets and earrings are not allowed for men, whatever the metal, because they copy women's adornment.
Gold is haram for men and silver is limited to a ring under one mithqal, plus fittings such as a belt buckle. Necklaces, bracelets, anklets and earrings for men are makruh tahrimi as imitation of women, even when made of other metals, leather or beads.
The four schools agree on this.
Explanation
Islam allows adornment and sees beauty as a blessing. For women, jewellery of every kind is lawful: gold, silver, pearls and precious stones. For men, the Prophet ﷺ cursed men who imitate women and women who imitate men, and he cursed the man who wears a woman's clothing. Since jewellery is mainly a women's adornment, the scholars of all four schools limit what men may wear.
What a man may wear. One silver ring under about 4.37 grams in the Hanafi school, with any stone. That is the whole of jewellery for a man. See can men wear a silver ring.
Diamonds and gems. A diamond set in a silver ring is fine for a man. Diamond studs, a diamond chain or an iced-out pendant are not, because the problem is the type of jewellery, not the stone.
Chains, bracelets and beads. A silver or steel chain, a bracelet or a beaded wristband worn as jewellery falls under the prohibition. Practical items are different: a watch, a medical alert band, a fitness tracker or a sports wristband are not jewellery. For earrings and other piercings, see are earrings haram for guys, and for gold, is gold haram for men.
The evidence
Quran, Surah Al-A'raaf 7:32
قُلْ مَنْ حَرَّمَ زِينَةَ ٱللَّهِ ٱلَّتِىٓ أَخْرَجَ لِعِبَادِهِۦ وَٱلطَّيِّبَٰتِ مِنَ ٱلرِّزْقِ ۚ قُلْ هِىَ لِلَّذِينَ ءَامَنُوا۟ فِى ٱلْحَيَوٰةِ ٱلدُّنْيَا خَالِصَةًۭ يَوْمَ ٱلْقِيَٰمَةِ ۗ كَذَٰلِكَ نُفَصِّلُ ٱلْءَايَٰتِ لِقَوْمٍۢ يَعْلَمُونَ ﴿٣٢﴾
Say, "Who has forbidden the adornment of Allah which He has produced for His servants and the good [lawful] things of provision?" Say, "They are for those who believe during the worldly life [but] exclusively for them on the Day of Resurrection." Thus do We detail the verses for a people who know.
Adornment is a blessing, within the limits set by the Sunnah.
Sahih al-Bukhari 5885
Sahih... لعن رسول الله صلى الله عليه وسلم المتشبهين من الرجال بالنساء ...
"The Messenger of Allah cursed the men who imitate women and the women who imitate men."
The reason men may not wear women's jewellery.
Sunan Abi Dawud 4098
Graded sahih... لعن رسول الله صلى الله عليه وسلم الرجل يلبس لبسة المرأة ...
"The Messenger of Allah cursed the man who wears women's dress and the woman who wears men's dress."
Applies the same rule to what is worn.
Sunan an-Nasa'i 5148
Graded sahih... أحل الذهب والحرير لإناث أمتي وحرم على ذكورها ...
"Gold and silk have been made lawful for the females of my community and forbidden to its males."
Jewellery of gold is lawful for women only.
Where the ruling is recorded
- al-Durr al-Mukhtar with Radd al-Muhtar, Kitab al-Hazr wa al-Ibaha, section on clothing. What silver a man may use, and imitation of women.
- al-Fatawa al-Hindiyya, Kitab al-Karahiyya, chapter on adornment, rings and jewellery. Rings, jewellery and adornment for men and women.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.