This is an early 20th century platinum and diamond American engagement ‘Eternity' ring, which the French call 'Alliance Americaine’. It bolsters a row of diamonds set in platinum.

Platinum, the hard but stunning material that is difficult to bend but beautiful to set, became highly popular in the Art Deco period, especially in the United States. The big jewellery houses like Tiffany's in New York and Cartier in Paris loved it and used it in their remarkable pieces.

Platinum became especially popular in 1920ties and 30ties America, hence the French name for the ring. If you got hitched, you had to have a platinum diamond engagement ring. In the English speaking world these rings are called 'Eternity rings'. 

And isn't that a fitting name?