What is the amount of zakat obligatory on gold possessed, and how is it calculated with varying carats (21 and 18), and what is the ruling on past years for which zakat was not paid after reaching the nisab?

Zakat is not obligatory on gold designated for adornment and personal use, though paying it is more cautious. Merely refraining from wearing gold or preferring not to wear it does not make Zakat obligatory on it, unless you intend it for savings. The determining factor is whether it is designated for use, regardless of the actual use itself.

If the gold is intended for savings, Zakat becomes obligatory after a full lunar year has passed since the intention of saving. As for gold designated for wearing, there is no Zakat on it as long as it is owned with the intention of personal use.

According to the opinion that Zakat is obligatory on jewelry designated for use, Zakat is only due for the period during which the conditions of obligation are predominantly met. For anything you are unsure about, the default is the exoneration of your liability.

When paying Zakat, it is necessary to differentiate between various gold finenesses. One-quarter of one-tenth (2.5%) of the value of each fineness should be given, either from the gold itself or from money, and the value at the time of payment is considered. If no money is available, a portion of the gold can be sold to pay the Zakat.