This is a fantastic example of Art Deco jewelry. It is an original antique art deco piece from the 1920's Germany. The center stone is a hybrid between old european cut and old mine cut, sometimes called an old mine brilliant. 

It is at least 2cts and about K-Si1. The diamond is surrounded by sapphires, this style was/is typically called a 'target ring'. Calibrated sapphires during the Art Deco period were often synthetic, which is likely the case for this ring. 

The white metal is platinum, and there is a faded stamp for platinum as well. Current size is 7.75 U.S. but it can be resized. Weight is 5.1 grams.