Some gold is stamped 999 and some 999.9. Here is what the difference is worth in grams and money, and when it is worth caring about.
Short answer: 999 gold is 99.9% pure and 999.9 gold is 99.99% pure. The difference is 0.09% of the weight, so on a one-ounce coin it is under a thousandth of an ounce, worth only a couple of units of currency at a round price of 2,000 per ounce. It rarely matters for value.
One troy ounce at 999 fineness holds 0.999 troy oz of pure gold, and at 999.9 it holds 0.9999 oz. The difference is 0.0009 oz. At a price of 2,000 per ounce, that is about 1.80, which is a small fraction of a typical premium.
Some refiners and mints aim for the highest purity as a mark of quality, and a few national coins are struck at 999.9. Higher purity does not make the metal more valuable per gram of pure gold, because you are paying for the pure gold in either case.
Use the gold melt value calculator with each fineness to see the difference for your weight. For the silver equivalent, read 999 vs 9999 silver. See also fineness.
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