Short answer
Wearing a gemstone as jewellery is fine. What is haram is believing that a birthstone, or any stone chosen by your birth month or star sign, brings luck, health or protection. That belief comes from astrology, and believing a stone itself has power is shirk. The four schools agree.
Gemstones are lawful adornment: for women in any jewellery, and for men set in a silver ring. Wearing a stone out of belief that it controls fortune or health, or choosing it by astrology, is haram, and believing the stone itself brings benefit or harm is shirk.
The four schools agree on this.
Explanation
Birthstones are a modern custom that links a gem to each month, often mixed with star signs and claims of healing or luck. Islam treats beautiful stones as one of Allah's gifts: the Quran mentions the ornaments people bring up from the sea to wear. The problem is never the stone. It is the belief attached to it.
The belief. Only Allah brings good and removes harm. The Prophet ﷺ reported that Allah said that whoever claims rain came because of a star has disbelieved in Him and believed in the stars. He also said that amulets worn for protection are shirk. Believing that a ruby brings wealth, that an emerald heals, or that a stone matched to your sign protects you falls under this. See astrology in Islam.
What is fine. A woman may wear any gem she likes because it is beautiful, including the stone for her birth month as a design choice with no belief attached. A man may set any stone, including a diamond or emerald, in his silver ring; see can men wear a silver ring. A birthstone necklace can be a gift if you leave out the luck and horoscope talk.
Aqeeq and "blessed" stones. Many Muslims wear aqeeq (agate) or other stones believing they carry special merit. The reports praising particular stones are not authentic, so wear them as plain adornment, not for blessing or luck. See is it sunnah to wear a ring.
If you once believed in these things, repent and move on; tawbah wipes it away.
The evidence
Quran, Surah Al-An'aam 6:17
وَإِن يَمْسَسْكَ ٱللَّهُ بِضُرٍّۢ فَلَا كَاشِفَ لَهُۥٓ إِلَّا هُوَ ۖ وَإِن يَمْسَسْكَ بِخَيْرٍۢ فَهُوَ عَلَىٰ كُلِّ شَىْءٍۢ قَدِيرٌۭ ﴿١٧﴾
And if Allah should touch you with adversity, there is no remover of it except Him. And if He touches you with good - then He is over all things competent.
Only Allah removes harm, not stones or stars.
Quran, Surah Faatir 35:12
وَمَا يَسْتَوِى ٱلْبَحْرَانِ هَٰذَا عَذْبٌۭ فُرَاتٌۭ سَآئِغٌۭ شَرَابُهُۥ وَهَٰذَا مِلْحٌ أُجَاجٌۭ ۖ وَمِن كُلٍّۢ تَأْكُلُونَ لَحْمًۭا طَرِيًّۭا وَتَسْتَخْرِجُونَ حِلْيَةًۭ تَلْبَسُونَهَا ۖ وَتَرَى ٱلْفُلْكَ فِيهِ مَوَاخِرَ لِتَبْتَغُوا۟ مِن فَضْلِهِۦ وَلَعَلَّكُمْ تَشْكُرُونَ ﴿١٢﴾
And not alike are the two bodies of water. One is fresh and sweet, palatable for drinking, and one is salty and bitter. And from each you eat tender meat and extract ornaments which you wear, and you see the ships plowing through [them] that you might seek of His bounty; and perhaps you will be grateful.
Gems and ornaments are lawful blessings to wear.
Sahih al-Bukhari 846
Sahih... أصبح من عبادي مؤمن بي وكافر ...
"This morning some of My servants became believers in Me and some disbelievers."
Crediting the stars with what Allah does is a form of disbelief, which covers astrology-based stones.
Sunan Abi Dawud 3883
Graded sahih... إن الرقى والتمائم والتولة شرك ...
"Spells, amulets and love charms are shirk."
Objects worn in the belief that they protect or bring good are forbidden.
Where the ruling is recorded
- al-Hidaya, Kitab al-Karahiyya, section on clothing. The ruling of a man's ring follows its band, not its stone, so any gem may be set in it.
- al-Durr al-Mukhtar with Radd al-Muhtar, Introduction, on which sciences are forbidden. Astrology is forbidden beyond what is needed for prayer times and the qibla.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. 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.