Start with the general rule the exception modifies. Section 408(m) treats an IRA's acquisition of a collectible, which the statute defines to include "any metal or gem," as a distribution of that amount, effectively banning collectibles from IRAs. Paragraph (3) then carves out an exception: certain gold, silver, platinum, and palladium coins and bullion may be held. The statute does not fix a purity percentage; it requires the bullion to meet the minimum fineness a regulated futures contract requires for delivery, which in practice means at least 99.5 percent for gold, 99.9 percent for silver, and 99.95 percent for platinum and palladium, and it requires the metal to be "in the physical possession of a trustee." Specific coins are also eligible by statute, including American Eagle coins, which is why the American Gold Eagle qualifies even though at 22 karat it is only about 91.7 percent pure, below the bullion fineness standard.
The physical-possession requirement is the point that most often goes wrong. The metal cannot sit in the owner's home safe or safe-deposit box; it must be held by the IRA's trustee or an IRS-approved depository. Promoters have marketed "home storage" or "checkbook" gold IRAs, often using a single-member LLC owned by the IRA, that claim to let the owner keep the coins. The U.S. Tax Court rejected exactly this structure in McNulty v. Commissioner (2021), holding that an IRA owner who took physical custody of American Eagle coins bought through an IRA-owned LLC had received a taxable distribution equal to the coins' cost, because she had unfettered control over them. Taking possession does not merely risk a penalty; it can be treated as pulling the metal out of the IRA entirely.
There is one more wrinkle worth knowing, covered in full on the gold-investing page: the same bullion that is eligible for an IRA can still count as a collectible taxed at the 28 percent maximum rate when held in a taxable account, because the rate rule reads the definition differently from the IRA rule. Inside the IRA, of course, ordinary IRA tax treatment applies instead. Whether owning gold at all makes sense for a given investor is a separate question, addressed on the gold-investing page; this page is about the account mechanics, and the general rules of a self-directed IRA, including the prohibited-transaction traps, live on that page.