Gold is one of the few metals that exist in a free state. In ancient China, there was a saying called "pure gold", which refers to pure gold. However, there is no such thing as absolutely pure gold. Even the purest gold will contain trace amounts of impurities such as copper and silver. The ancients also knew this, so there is a saying "Gold is not pure, and no one is perfect", which means there is no absolutely pure gold in the world. This is used to metaphorically say that nothing in the world is perfect, and we can't expect a person to have no flaws or mistakes at all.
So, what kind of gold can be considered pure gold? Modern gold purity standards have been standardized. In 1914, international standards defined 24K gold as 100% pure gold, and the gold content of 1K gold is 100%/24 = 4.166666%. Thus, we can calculate the gold content of common 18K gold: 18 × 4.166666% = 75.0%.
However, even gold labeled as 24K is not likely to be 100% pure, which is just a theoretical value. Generally, gold with a gold content of not less than 99.0% is called "pure gold" and is marked with "pure gold", "99 gold" and other marks; and gold with a gold content of not less than 99.9% is called "thousand-pure gold" and is marked with "thousand-pure gold", "999 gold" and other marks. Although thousand-pure gold is the highest gold content standard in gold products, it is obviously not 100% pure. Compared with single-crystal silicon used in semiconductor manufacturing, which has a purity of 99.9999% or even 99.9999999%, thousand-pure gold's purity is nothing compared to it.