I heard that gold is much cheaper in China than in Taiwan.
About calculating the value of a 5-gram piece of gold jewelry, it depends what kind of jewelry it is. Different kinds of jewelry have different values depending on how much someone is willing to pay for it, which depends on how well it is made, how attractive it looks, how intricately it is carved, etc. In addition, you have to look at the quality of the gold. For example, is it shiny and well-polished or dull and scratched up or worn down?
So the “spot price” for gold has nothing to do with the value of a piece of gold jewelry. The “spot price” is just the current price for trading gold bars or bullion or ingots. Certain commemorative coins are also traded on commodity markets, but the trading price of the coins is always much higher per ounce than the trading price of gold bullion.
But you are correct that there are 31.1 grams per troy ounce. Don’t forget that “ounces” always means “troy ounces” when dealing in the precious metals market. (There are only 28.35 grams in a “regular” ounce, which is technically called an “avoirdupois ounce”.)
The “London Fix Price” is in US dollars because the US dollar is the standard currency for international trading. The Euro is also an international trading currency, but they don’t use Euros in Britain. And I know that gold is also traded on the “New York Commodity Exchange”, as well as lots of other places.