There is no zakah on precious stones unless they are for trade
Question

What is the rate of zakah that must be paid on precious stones such as diamonds, as they are not gold?.
Praise be to Allah.The majority of scholars are of the view that no zakah is due on precious stones unless they are for trade. In their opinion, zakah is only due on gold and silver. 
Imam Malik said in al-Mudawwanah (1/431): 
No zakah is due on gems, pearls or amber. 
Al-Shaafa’i said in al-Umm: 
No zakah is due on the things that women wear for adornment, or that are kept by men and women, such as pearls, chrysolite, sapphires, rubies, precious stones extracted from the sea, etc. zakah is only due on gold and silver. 
Al-Nawawi said in al-Majmoo’: 
Apart from gold and silver, no zakah is due on gems such as sapphires, turquoise, pearls, rubies, emeralds, chrysolite, iron, brass, copper and glass, no matter how beautifully they are made or how high their value. No zakah is due on musk and amber either. 
There is no dispute regarding any of these things in our view. This is the view of the majority of scholars of the salaf and others. Ibn al-Mundhir and others narrated that al-Hasan al-Basri, ‘Umar ibn ‘Abd al-‘Azeez, al-Zuhri, Ibn Yoosuf and Ishaaq ibn Raahawayh said that the khums must be paid on amber. Al-Zuhri said: The same applies to pearls. Our companions narrated that ‘Abd-Allah ibn al-Hasan al-Anbari said: The khums must be paid on everything that is extracted from the sea, apart from fish. Al-‘Anbari and others narrated two reports from Ahmad, one of which is like the view of the majority, and the other says that he said that zakah must be paid on everything that we have mentioned, if its value reaches the nisaab – even musk and fish. Our evidence is as follows: 
1-The basic principle that no zakah is due except in cases where there is shar’i evidence
2-It was narrated in a saheeh report that Ibn ‘Abbaas (may Allah be pleased with him) said: No zakah is due on amber, rather it is something that the sea has thrown up
What we have mentioned is the conclusion based on the evidence concerning this matter. With regard to the hadeeth narrated from ‘Amr ibn Shu’ayb, from his father, from his grandfather, that the Prophet (peace and blessings of Allah be upon him) said: “there is no zakah on stones,” this is very weak (da’eef jiddan). It was narrated by al-Bayhaqi who stated that it is weak. 
Shaykh Ibn Baaz was asked (14/121): 
Nowadays there are many types of jewellery such as diamonds, platinum etc which are made for wearing etc. Is any zakah due on them? What if they are made into vases and vessels for decoration or for use? Please advise us, may Allah reward you. 
He replied: 
If the jewellery is made of gold or silver, then zakah is due on it, if it reaches the nisaab and one full hijri year has passed, whether it is for wearing or for lending, according to the more sound of the two scholarly opinions, because of the saheeh ahaadeeth which have been narrated concerning that. If they are not made of gold and silver – such as diamonds, carnelians, etc – then no zakah is due on them, unless they are intended for trade, in which case they come under the general heading of trade goods, so zakah must be paid on them as in the case of all other trade goods. It is not permissible to use vessels of gold and silver even if they are only for adornment, because using them for adornment may lead to using them for eating and drinking. It was narrated in a saheeh hadeeth that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Do not drink from vessels of gold and silver and do not eat from plates of gold and silver; they are for them – meaning the kuffaar – in this world and for you in the Hereafter.” Saheeh – agreed upon. 
The one who has acquired such things has to pay zakah on them and also repent to Allah. He also has to change them into something that does not look like vessels, such as jewellery etc. 
He also said (14/124): 
Jewels that are not gold, such as diamonds, are not subject to zakah, unless they are intended for trade. 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: 
What is the ruling on keeping jewels such as diamonds? Is zakah due on them? Is the ruling on diamonds the same as the ruling on gold and silver? 
He replied: 
Keeping jewels to use them is permissible subject to the condition that that does not reach the level of extravagance. If it reaches the level of extravagance it is not allowed, based on the general principle that forbids extravagance, which oversteps the limit set by Allah. Allah says (interpretation of the meaning): 
“and waste not by extravagance. Verily, He likes not Al-Musrifoon (those who waste by extravagance)”
[al-An’aam 6:141]
If keeping these jewels such as diamonds etc does not reach the level of extravagance, then it is permitted, because of the general meaning of the verse in which Allah says (interpretation of the meaning): 
“He it is Who created for you all that is on earth”
[al-Baqarah 2:29]
No zakah is due on them unless they are prepared for trade, in which case they come under the same ruling as all other trade goods. 
Fatawa al-zakah, p. 97.