Is Zakat obligatory on ornamental gold that has not been worn for 20 years, knowing that Zakat was paid on it once at the beginning of the marriage, and if it is obligatory, how is the Zakat for the past years calculated?

The madhhab of the majority of scholars is that jewelry prepared for permissible use is not subject to zakat at all. As long as this jewelry is prepared for use, there is no zakat on it, even if it is not used. Therefore, a woman is not obliged to pay zakat on any of her jewelry for any of the past years. However, if she wishes to pay zakat on this gold for the past years as a precautionary measure, she may do so.