1. If a person possesses 612.35 grams of silver or 87.479 grams of gold or any currency that equals the value of this amount of gold or silver, and this wealth remains in his possession for a complete year, then on the expiry of this year it will be wajib on him to give zakaat for it. If his wealth is less than this, zakaat will not be wajib. If it is more than this, zakaat will still be wajib.
This is from: http://www.central-mosque.com/fiqh/Zakat.htm

So I looked up the current values of a gram of gold and a gram of silver; I did the math and came up with the nisaab values.

But the nisaab value are different.
If you do the math for silver, the nisaab = $636.84
If you the math for gold, the nisaab = $4,670.5

The two value are so far apart! Im sure I did the math right. What nisaab do you choose if the two are different? Or am I going about this all wrong?

NOTE: im not even eligible to pay zakah, I was just curious to see what the values would be.