If the gold reaches the minimum threshold at which zakaah becomes due (the nisaab), which is 85 grams, and one full hijri year has passed since it was acquired, then zakaah must be paid on it, by giving one quarter of one tenth of it, or of its value. What is meant by its value is the price for which it would be sold used at the time when zakaah is due. This is usually less than the price for new gold.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: Based on this, a woman should work out the value of the gold that she has, whether that is the price for which she bought it or less, or more, so she should work out its value as used gold then pay one quarter of one tenth, i.e., one in forty, or two and a half riyals out of every hundred, or twenty five out of every thousand, and so on. The way to do that is to divide the value by forty, and the result is what is to be paid in zakaah. In this manner she will discharge her duty and be ransomed from the punishment of the Fire, and nothing will harm her.
He was asked: Is the zakaah of jewellery based on the purchase price or the price each year at the time when zakaah is paid?
He replied: Zakaah on jewellery must be paid each year and cannot be based on the purchase price, rather it must be based on the price when the full year has passed. If a woman buys gold for ten thousand riyals, and when the year has passed it is only worth five thousand riyals, then she should only pay zakaah on five thousand riyals. The converse is also true: if she buys gold for five thousand riyals then when the year has passed it is now worth ten thousand riyals, then she should pay zakaah on ten thousand riyals, because that is the time when zakaah becomes obligatory. And Allaah is the Source of strength.
From Majmoo’ Fataawa Shaykh Ibn ‘Uthaymeen, vol. 18, question no. 18, 58.
And Allaah knows best.