How is Zakat calculated on 18 and 21-carat gold that contains non-precious stones? Is it calculated based on the weight of the gold only, without the weight of the stones? And how is the nisab determined for different carats of gold?

Zakat is obligatory on gold if it reaches the nisab (85 grams) and one hijri year has passed over it. Pure gold (24-karat) is what Zakat is calculated upon. As for adulterated gold (mixed with other metals), Zakat is calculated only on the amount of pure gold within it. The pure gold is calculated by multiplying the number of grams by the karat and dividing the result by 24. However, if the gold is intended for trade, it is appraised at the beginning of each year, and one-quarter of one-tenth of its value is to be given as Zakat if it reaches the nisab. Any other metals mixed with it are included because they are sold along with it.