Regarding whether it is the raw gold in jewelry that will be considered while measuring it for zakah or the weight of the whole jewelry,  Dr. Monzer Kahf, Scholar in Islamic Economics and a Financial Expert, stated:

“In principle, what matters is the amount of pure Gold in the jewelry not the gross weight. At the time of the Prophet, peace be on him, gold was used as currency and it was used in different forms: at least three or may be four forms are reported: fine powder-like, irregular pieces like small rocks, regular bullions and minted currency. Comparing them together puts the Nisab in pure gold because the dinar was a weight as much as it was a minted coin.”