Zakah on Gold and Silver
The Nisab (quotum) of Gold is 20 mithqal (1440 grains of barely) which is 84.875 grams of pure gold, so about 85 grams for pure gold.
This is evaluated to 86 grams for 24karat gold, and 97 grams for 21karat gold, and 114 grams for 18karat gold. And for other karats, one needs to evaluate how much of pure gold it pertains and based on that will give the zakah if there is about 85grams of pure gold or more.
The Nisab of Silver is 200 dirham (10080 grains of barely) which is 594.125 grams of pure silver, and some evaluated it as 595 grams of pure silver.
The Zakah is due after the elapse of one lunar year on the full Nisab. One has to give in as zakah from that 1/4 of 1/10 of the market value of these, i.e., 1/40th which is 2.5% of the market value. And according to some renowned scholars, one may evaluate the %2.5 of the nisab with monetary value and give the money as Zakah to any of the 8 types of people mentioned in al-Qur'an.
Zakah on Curreny - Zakatul-Maal
According to Imam Abu Hanifah, Zakah is due on money currency. If one's own money has the full Nisab (quotum), and a full year had lapsed on that amount, then the due Zakah is 2.5%. The nisabe of money would be similar to the value of the nisab of Gold or Silver.
We know that the value of gold and silver changes. Hence, we can say that the minimum nisab is 3 oz = 86 grams of gold or 21 oz = 595 grams of silver. One has to check the dollar value and pay it after they have had this gold (or silver) for a whole lunar year. The Hanafiyys have a certain way of calculating this Zakah. However, if one pays 1/40 = 2.5% one will be paying MORE than required. Hence, the person intends the extra amount to be Sadaqah - charity.
The hanafiyys say that one evaluates by what is more beneficial to the poor people. If your money constitutes a nisab of silver but not gold, then you must pay zakah according to the value of silver.