Shaykh Bin Baz was asked:
“Is it allowed for men to wear a diamond watch?”
“I do not know of any harm in it. Diamond is from the types of silver while gold is prohibited. Gold is prohibited for men and permitted for women. As for diamonds, then I do not know of anything wrong in it and the origin is permissibility as it is permissible to wear a silver ring. As for if the diamond is more expensive than gold and he loves to abstain from it, then there is no harm. The origin is permissibility because prohibition is in the hand of the legislator, not in the hand of the people. The legislation prohibited gold and not silver and not iron. Like this the Prophet (ﷺ) said:
الْتَمِسْ وَلَوْ خَاتَمًا مِنْ حَدِيدِ
Look for something (as a dowry) even if it is an iron ring
[Al-Bukhari no. 5135]
So if he wears an iron ring, a silver ring, or a yellow copper ring, then there is no harm. What is prohibited for the men is a gold ring. Diamond is not gold and it is not called gold, even if it is expensive.” End translation.
Faisal Ibn Abdul Qaadir ibn Hassan