Somewhere in America right now, a 74 year old is selling a full ounce of gold because he needs eleven thousand dollars.
He does not want to sell the whole ounce. He has no choice. It is the smallest thing in the account.
So the extra proceeds sit in cash inside the IRA, earning nothing, until next year, when the same thing happens again.
Fractional gold coins exist to solve exactly this problem, and almost nobody thinks about them until it is too late to buy them cheaply.
Yes, they are allowed. Half ounce, quarter ounce, tenth ounce American Gold Eagles are all IRA eligible, along with fractional coins from several other mints.
The question is not whether you can. It is how many, and when, and what that convenience costs you.