Most of the US gold reserve consists of low-purity bars that are not suitable for global settlements.

According to an article from the Mises Institute, the majority of the US gold reserve stored at Fort Knox is made up of "non-standard" low-purity bars that do not meet international settlement standards according to LBMA criteria, which require a purity of at least 995 out of 1000.
Only 17% of Fort Knox's bars meet these standards, while 64% have a purity ranging between 899 and 901, making most of the US reserve practically unacceptable in global markets.
Officially, the US reserve amounts to 8,133.5 metric tons, half of which is at Fort Knox, with the rest distributed between Denver, West Point, and the Federal Reserve in New York.
Despite this enormous figure, gold is still legally priced at only $42.22 per ounce, a price that has not been updated for decades and does not reflect any market movement.
The biggest problem lies in the absence of any actual auditing since the 1970s.
The "audit" in 1974 was merely a media spectacle, where only one of 15 vaults was opened, without matching serial numbers, checking purity, or verifying ownership.
Worse still, there is evidence of vault seals being broken and re-sealed later without auditing, along with some records being lost.
This issue stems from Executive Order 6102 issued by Roosevelt in 1933 to criminalize private gold ownership, during which the government confiscated large quantities of 90% purity gold coins, melted them down, and cast them into bars.
At that time, the government paid $20 per ounce, then six months later, it devalued the dollar by making the ounce worth $35, resulting in a 40% loss for citizens.
This historical legacy explains why today there is so much non-standard gold in American vaults.
In contrast, other countries have taken practical steps to address this problem;
the French central bank recently sold 129 tons of similar non-standard gold stored in New York and replaced it with high-quality bars kept in Paris.
In the United States, Senator Mike Lee has introduced a bill calling for a comprehensive audit and re-refining of the non-standard bars, a process that could take years.
The conclusion of the article is that the world's largest official gold reserve may, in fact, be a dilapidated legacy of confiscation and monetary manipulation, and the lack of transparency for decades makes claims about its size and purity seriously questionable.