All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
If you use diamonds merely for the purpose of adornment, then zakat is not due on it. However, if they are dedicated for trade, then you must pay zakat on them if their value reaches the Nisaab, by itself or in addition to other trade goods or money, after the lapse of one lunar year.
Allaah Knows best.