Praise be to Allah.
Jewellery made of gold that is mixed with other metals, such as copper, is not subject to zakah unless the pure gold content of it reaches the nisab. If it only reaches the nisab when the copper is included, then no zakah is due on it.
An-Nawawi (may Allah have mercy on him) said: Our view is that no zakah is due on gold alloys or silver alloys unless the fine-metal components reach the nisab. This is the view of the majority of scholars."(Al-Majmu‘ 5/505).
Ibn Qudamah (may Allah have mercy on him) said: If someone owns an alloy of gold or silver that is mixed with another metal, then no zakah is due on it unless the gold or silver component reaches the nisab, because the Prophet (blessings and peace of Allah be upon him) said: “No zakah is due on anything less than five uqiyahs of silver.”(Al-Mughni 2/319).
But how can we work out the weight of the pure gold in it?
The answer is:
We should consult experts concerning that, namely goldsmiths and jewellers, who are able to determine whether the pure gold reaches the nisab or not.
If it reaches the nisab, then zakah is due on it; if it does not reach the nisab, no zakah is due on it.
And Allah knows best.