The Zakat of Silver and Gold
The Zakat of silver and gold, whether in currency or not, is obliged by the Sunnah and Ijma’a as-Sahabah. As for the Sunnah, this is due to what Abu Hurayra narrated: The Messenger of Allah (saw) said: “No owner of gold or silver who fails to give their due right except that he will have sheets of fire made for him on the Day of Judgement.
They will be heated in the Hellfire then used to burn his sides, forehead and back. Whenever they cool they are reheated to him in a Day whose length is 50,000 years long until it is judged between the people and he is shown his path, either to Paradise or to the Fire” (narrated by the five except At-Tirmizi). Also he (saw) said: “Whoever is given wealth by Allah and fails to pay its Zakat then his wealth will be presented to him on the day of judgement as a bald snake with two spots above its eyes and it surrounds him on the Day of Judgement.
Then it will hold him by two sides of his jaw and it will say: I am your wealth, I am your hoarded treasure.” Then he recited: “Let not those who covetously withhold of the bounties which Allah has given them of His grace think that it is good for them. Soon shall what they covetously withhold be tied to their necks like a twisted collar on the Day of Judgement.” [Al-Imran: 180] (narrated by the five except Abu Dawud).
As for Ijma’a, the companions had a consensus without anyone disagreeing upon the obligation of Zakat on silver and gold.
The Nisab Amount of Silver
The least amount of silver upon which Zakat is obliged is five ounces due to his  saying: “There is no Sadaqah on anything less than five ounces” (agreed upon). Its amount is counted as 200 Dirhams because each ounce is 40 Dirhams. From Ali b. Abi Talib who said: “Five Dirhams (are due) in every 200 Dirhams.” Muhammad b. Abdur Rahman al-Ansari narrated that in the book of the Messenger of Allah (saw), and the book of ‘Umar ibn Al-Khattab, on Sadaqah (there was stated): “Nothing is taken from silver until it reaches 200 Dirhams.” Thus if silver is less than 200 Dirhams, even by just one Dirham, then no Zakat is obliged upon it, due to its being less than five ounces. The Messenger of Allah (saw) did not oblige Zakat in what is less than five ounces.
The Dirhams against which the Nisab is considered are the Shari’ah Dirhams.
Every ten of them are equivalent to the weight of seven Mithqal of gold.
Each Dirham is equivalent to seven-tenths of a mithqal. This is the Shari’ah Dirham against which is measured the Nisabs of Zakat, the value of Jizya, blood-monies, the Nisab of cutting for stealing etc.
The dirham’s weight in grams utilised today is 2.975 grams, so the weight of the Nisab of Zakat in silver-which is 200 Dirhams-becomes 595 grams. It includes the minted silver as currency as well as the dust (Tibr) or ingots (Sabaik). If the silver is mixed with lead, copper or other types of minerals such that its pure content reaches the Nisab of silver, then Zakat is obliged and taken from the amount of pure silver in it.
The Amount of Zakat Obliged on the Nisab of Silver
If silver reaches the Nisab of Zakat and a full year passes over it, then quarter-tithe is obliged in it i.e. five Dirhams are obliged in the Nisab which is 200 Dirhams. This has been established by the Sunnah. On the authority of Abu Bakr As-Siddiq it is narrated that the Prophet (saw) said: “Quarter-tithe in the Riqqa.” Ar-riqqa is the minted silver. Bukhari narrated the Prophet (saw)’s saying: “Bring the Sadaqah of the Riqqa as a Dirham for every forty, and there is nothing due on one hundred and ninety.” From Muhammad b. Abdur Rahman al-Ansari who said that in the book of the Messenger of Allah (saw), and the book of ‘Umar, on Sadaqah there is the following: “Nothing is taken from silver until it reaches 200 Dirhams.” The five Dirhams obliged in the Nisab of Zakat are equivalent to 14.875 grams, as the dirham’s weight is 2.975 grams.
The Nisab amount of Gold and what Zakat is Obliged in it
The least amount of gold in which Zakat is obliged is 20 Dinars. If it is less than 20 Dinars, even by only one (Qirat), then no Zakat is obliged in it. Ali b. Abi Talib was reported to have said: “One-half Dinar in every 20 Dinars, and one Dinar (is due) in every forty Dinars.” From Amru b. Shuaib from his father from his grandfather from the Prophet (saw)  who said: “There is no Sadaqah on whatever is less than 20 Mithqal of gold.” Gold is counted together in all its parts, from the unbroken to the broken, the coined (Madhruba), to the dust (Tibr) or ingots (Sabaik), all is counted as one with regards to the amount.
As for what is obliged in the Nisab of gold as Zakat, this is quarter-tithe (2.5%) i.e. one-half Dinar in the Nisab, which is 20 Dinars, and one Dinar in forty Dinars due to the above-mentioned Hadith.
Whatever is more than the Nisab of gold and silver is subject to the same assessment, i.e. quarter-tithe, whatever the increase, whether small or large.
This is different from the rule of livestock where what is between two Nisab amounts is forgiven such that there is no Zakat in it. As for gold and silver, there is Zakat in every increase upon the Nisab, and there is only one Nisab.
Whatever is more than the Nisab takes the rule of the Nisab and its Zakat is collected i.e. quarter-tithe is taken from it.
The weight of the Nisab of gold of 20 Dinars is 85 grams of gold. The weight of the half-Dinar obliged in the Nisab of Zakat is 2.125 grams gold as the Dinar’s weight is 4.25 grams of gold.
No Zakat is obliged on the Nisab of gold and silver except if a full year passes over it, where the Nisab is complete from the beginning to the end of the year. On the authority of Aisha, she said: ‘I heard the Messenger of Allah (saw) say: “There is no Zakat in property until a year passes over it.’” As for a person who owns less than the Nisab of gold or silver at the beginning of the year then he gains wealth so that he completes the Nisab before the end of the year, his year begins from the time the Nisab is completed, such that when a full year passes, only then is Zakat obliged on it.
However if the Nisab of gold and silver is complete from the beginning of the year and it increase during the year then if the increase emanated from trade it is added to the original amount (Asl). This is because the year of the increase is considered the same as the year of the original amount, for the increase is of the growth of the original money, and is of its own type, so it has the same rule.
However, if the increase was of the Nisab’s type but it did not come through the means of growth such as if it came from inheritance or donation, then a full year must pass upon this increase. It is not added to the original amount nor does it take the rule of its year. Similar to this is when the increase comes from a different type of wealth like an increase in livestock. This is not added to gold and silver and a full year must pass over it before Zakat is obliged when it reaches the Nisab. The Nisab of gold is not completed by silver, nor is the Nisab of silver completed by gold, as they are two different types. This is similar to the fact that the Nisab of dates is not completed by raisins nor is the Nisab of camels completed by cattle, because the Hadith says: “There is no Sadaqah in what is less than five ounces.”
Moreover, the Messenger (saw) made them two different types and allowed quantitative differences between them in exchange transactions.