Should zakaah on jewellery be based on the price for new gold or used gold?

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.