Is zakat obligatory on the entire quantity of gold, or on what exceeds the nisab, if the saved quantity of gold exceeds the nisab?

Zakat is obligatory on all gold if it reaches the nisab, which is 85 grams. So, if someone possesses, for example, 100 grams, they are obligated to pay zakat on the entire 100 grams, due to the Prophet's (peace be upon him) saying: "If it is two hundred dirhams, then five dirhams are due on it, and whatever exceeds that, then it is calculated proportionally." The obligatory amount is a quarter of a tenth (2.5%), to be given from the gold itself or its value in cash.