The Atlas · Platinum mark
PLAT, PT, 950, PT950 and PLATINUM all mean platinum, normally 950 parts per thousand. IRIDPLAT means platinum alloyed with 10% iridium, and it is one of the most useful marks in an estate box because it points hard at American work made between roughly 1910 and 1940.
What does PLAT or IRIDPLAT mean on a ring? — answered here in full, free, with nothing asked of you.
Platinum is the metal that gives away the era before any stone does. It came into jewellery use around 1900, became the setting metal of the Edwardian and Art Deco periods because it could hold a diamond in a knife-thin claw, and was then banned from non-military use in the United States in 1942 — which is why white gold appears everywhere after the war. A platinum mount with a filigree gallery and millegrained edges is very probably pre-war, and that dating is worth real money on a piece that would otherwise read as a plain diamond ring.
You can also feel it. Platinum is roughly 60% denser than sterling silver and noticeably heavier than 18 karat gold, so a platinum ring in the palm feels wrong for its size — heavy in a way that surprises people. It also does not tarnish; a white metal ring that has stayed bright for eighty years while the silver beside it went black is a candidate before any mark is read.
The mark itself is often nearly gone. Platinum is worked thin, and eighty years of wear on the inside of a band takes a shallow PLAT stamp down to a shadow. This is one of the few cases where the absence of a mark means very little, and where the construction — millegrain, pierced galleries, knife-edge shanks, calibré-cut side stones — carries the argument instead.
The stamp is present and legible: this metal is declared platinum. IRIDPLAT additionally declares the 10% iridium alloy used almost exclusively by American makers of the period.
For IRIDPLAT, American manufacture in roughly 1910 to 1940, which is the Edwardian and Art Deco window. For a bare PLAT with millegrained edges and a pierced gallery, the same period; for PT950 in modern lettering, a piece made after about 1980.
Whether the white metal is platinum at all when the stamp is worn away, and whether a stone set in it is a natural diamond, a synthetic, or white sapphire. Both questions need a bench: a specific gravity test for the metal, a gemologist for the stone.
Next step
Weigh the ring on a kitchen or jewellery scale and photograph the edge of the setting side-on. Weight against size separates platinum from silver and white gold in one number, and the side view shows millegrain and gallery work that dates the piece independently of any stamp.
White gold, rhodium plated. Lighter in the hand, and the plating wears yellow at the back of the shank where platinum stays white.
Palladium: platinum's much lighter, much cheaper sister metal, common in the 1940s substitute period and again after 2005.
Silver. Softer, and tarnishing in the crevices where a cloth cannot reach — the fastest visual separation from platinum.
White gold spelled out. Same metal as 18K W, different notation.
Free, and nothing asked of you
One mark answers one question. A box answers the real one — which pieces deserve money and expertise, and which nine are costume. Email the photographs to the house inbox: the whole box laid out, then any stamps you can find. We read them and reply by hand.