Is Zakat obligatory on gold and diamonds prepared for wearing, especially if some of it was a gift and one might be forced to sell some of it to benefit from its price, and what is the ruling on selling or gifting gifts that are not worn?

Jewelry other than gold and silver (such as pearls and diamonds) for adornment is not subject to Zakat (definition). However, if it is for trade, then it is subject to Zakat on trade goods. It is permissible to sell or gift a present because it has become the property of the one to whom it was gifted, and thus he has the right to dispose of it in any lawful manner. However, if gifting it away would upset the giver, and there is no need to sell or gift it, then it is better not to dispose of it out of consideration for his feelings.