Finished with a polished surface that catches the light beautifully. Chain Necklace — Length & Weight Guide. 16" 2.48 GM 1.65 mm. 18" 2.78 GM 1.65 mm. 20" 3.09 GM 1.65 mm. 24" 3.71 GM 1.65 mm. Images may be enlarged to show fine details.