hello i recently purchased a necklace and pendent and do not understand what these marks mean on the chain its says 925 so at first i though it was silver but on the reverse side of the mark it has a star then 620 ar anyone know what these markings mean? any help or info would be great thank you! also i should mention that it is a 20" 4mm curb chain and weighs 15.4 i researched similar sterling silver ones and they weigh around 11-12g so mine is pretty heavy