Why was a gold ring forbidden for men, yet a diamond ring and other jewels permitted for them?

The essence of servitude to Allah Almighty dictates that the accountable person submits to His حكم (rule/decree), whether the reason behind the ruling is apparent to him or not. Scholars have mentioned, regarding the wisdom behind the prohibition of gold for accountable individuals, except for the permissible adornment of women with it, that it involves extravagance, pride, and arrogance. As for your query regarding the permissibility for men to adorn themselves with precious jewels while gold is forbidden, and that if it were due to extravagance and arrogance, those jewels would also be forbidden, the answer is that this issue is not a matter of consensus. Some scholars have prohibited men from adorning themselves with these jewels. And according to the opinion of permissibility, the difference lies in what they mentioned concerning the permissibility of utensils made from precious jewels alongside the prohibition of gold utensils.