Praise be to Allah.
There is nothing wrong with a woman having a diamond fitted on her tooth for cosmetic purposes, because the basic principle is that it is permissible. We do not know of any reason why she should not do that, unless she is doing it for cosmetic purposes in ways that are forbidden because she will show it in front of non-mahram men , or this diamond is fitted by a non-mahram man, because there is no case of need which would make it permissible for her to uncover in front of him in that case, or if the matter reaches the level of extravagance and wasting money.
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) has issued a fatwa stating that it is permissible for a woman to have gold teeth fitted if that is customary for cosmetic purposes among women.
For more, please see these answers: 110357 , 112079 , 82409 , 21255 , 98519 , and 82647
And Allah knows best.
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