Question

Why gold is not weighed by a spring balance

Answer 1

The price of 10g of gold is around rupees 30,909 (i.e. a lot money). So we need a lot of accuracy(to milligrams) in calculating exactly how much we are weighing. Spring balances aren't all that accurate and cannot provide us with as many decimal places as we'd like. So more sophisticated/sensitive/accuracy have to be used in case of measurements related to gold.

and also we do not take weight of gold because weight of a substance is not constant and varies with places: like a body will weigh more in the poles than in the equator.