What “950 Plat” and “900Pt” actually claim
A platinum stamp is a statement of parts per thousand. “950 Plat” or “950Pt” claims 950 parts in every thousand, by weight, are pure platinum — 95%. “900Pt” claims 90% and “850Pt” 85%. The abbreviations Plat and Pt are interchangeable, and both are named in the US rule. The remaining metal can be other platinum-group metals such as iridium, ruthenium or palladium, or a base metal such as cobalt or copper.
In the United States the governing text is 16 CFR § 23.6, part of the Federal Trade Commission’s Guides for the Jewelry, Precious Metals, and Pewter Industries. It sets three thresholds. At 950 parts per thousand or more, a piece may simply be called “Platinum”. At 850, 900 or 950, the word may be used with the number in front of it. Below 500, the word may not be used at all.
Alloy marks such as 600Pt.350Ir and 585 Pt 415 Co
Between 500 and 849 parts per thousand, the US rule turns on whether the rest of the metal is also platinum-group. If platinum plus the other platinum-group metals reach 950 parts per thousand, the piece may be marked as platinum provided every platinum-group constituent carries its own number: the rule’s examples are “600Pt.350Ir.” and “550Pt.350Pd.50Ir.”.
If the total does not reach 950 — typically because the balance is cobalt or copper — the stamp must give the full composition, as in the rule’s example “585 Pt., 415 Co.”, and the seller must disclose that the metal may not have the attributes of traditional platinum, unless they hold competent scientific evidence that it does not differ materially in durability, lustre, density, scratch and tarnish resistance, hypoallergenicity, ability to be resized or repaired, and retention of precious metal over time. The rule singles out “600Plat.” on its own as a misleading marking, which is why the decoder above flags a bare number under 850.
The UK reads platinum differently
In the United Kingdom, the Hallmarking Act 1973 approves only four platinum standards: 850, 900, 950 and 999. Palladium was brought under the Act in 2009 with three: 500, 950 and 999. A UK hallmark is struck by an independent assay office after the metal has been tested, and it always shows one of those numbers alongside a sponsor’s mark identifying who submitted the piece and an assay office town mark. Platinum articles under half a gram, and palladium articles under one gram, are exempt from compulsory hallmarking.
The practical difference is who checked the claim. A UK platinum hallmark means the state-appointed assay office tested it. A US “950 Plat” stamp means the maker says so and the FTC rule holds them to it, but no office tested it before sale. The marks can look alike; the assurance behind them is not the same.
Palladium, and what the stamp does not tell you
For palladium, the US rule names the abbreviations “Pall.” and “Pd.” but sets no numeric minimum; the general prohibition on misrepresenting composition is what applies. This page reports that absence rather than filling it with a figure the rule does not contain.
No stamp says how much a piece weighs, whether it is plated, or what it would fetch. Platinum-coated and rhodium-plated pieces are a common source of confusion: the US rule bars calling a coated item platinum unless the description makes clear it is only a coating.