A gift tax is due and payable by the donor and if your "friend" is not a US citizen or resident, then they do not have to file a gift tax return and they are not liable for gift taxes. The recipient does not pay the tax (except in certain situations, of which this is not one if all is as you stated).
Of course, it does raise issues as to why the transaction consists of a gift of diamonds and not funds transferred through the normal course but that is another matter entirely. But if it truly is simply a gift of diamonds, and nothing more, then no tax is payable by you.
Eric P. Rothenberg, P.C.
ORSI ARONE ROTHENBERG
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Gifts whose value is higher that USD100 are subject to the payment of customs duties. I believe in such situation CBP will assess a duty rate on the fair market value of the gift.
Diamonds are classified within chapter 71 of the Harmonized Tariff Schedule; check out the tariff first, then confirm with CBP at the following: https://help.cbp.gov/app/home/search/1
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