The minimum threshold for gold and silver is based on pure gold and silver
Question

If the grade of gold that is used in working out the minimum threshold is 24 karat, what is the grade of silver if I want to work out the minimum threshold of my wealth on the basis of silver, and the silver is of more than one grade?
Praise be to Allah.If the amount of pure gold or pure silver reaches the minimum threshold, then zakah is due on them.
Similarly, if the amount of pure gold or silver in an alloy or adulterated item reaches the minimum threshold, zakah must be paid – when one full hijri year has passed.
Ash-Shawkani (may Allah have mercy on him) said: The words “not adulterated” are correct, because if gold and silver are adulterated with metal that is not gold or silver, this additional metal is not subject to zakah, so the amount of adulterating metal is waived, and zakah is only given on the pure gold or silver, whether the type of gold and silver is of high quality or of low quality, because low-quality gold is still called gold, and low-quality silver is still called silver.
End quote from as-Sayl al-Jarrar, p. 234.
See also the answer to question no. 128167.
Secondly:
The minimum threshold for zakah on pure (24 karat) gold is 85 grams, when the purity of the gold reaches the level of 999 or 1000, which is the highest grade of purity for gold according to experts in the field.
With regard to silver, we have noted previously, in the answer to question no. 64, that the minimum threshold for silver is approximately 595 grams. So whoever owns this amount, in whatever form it is, or owns the value thereof in cash or trade goods, must give zakah on it when one full hijri year has passed.
According to specialists, pure silver is that of which the purity level reaches 1000.
If the wealth reaches the minimum threshold, whether it is currency – gold or silver, pure or mixed – or it is trade goods, shares and the like, and one full hijri year has passed, then zakah becomes due on it.
What counts is working out the minimum threshold, which is to be based on what we have discussed above, not the grade of the gold or silver. Gold or silver may reach the minimum threshold when it is mixed with an alloy, and it may not reach the minimum threshold when it is in its pure form.
You can find out the minimum threshold by asking specialists about the price of a gram of pure silver. Then if your wealth reaches the price of 595 grams of pure silver, then it has reached the minimum threshold.
For more information, please see the answers to questions no. 145770 and 159371.
And Allah knows best.