Finished with a polished surface that catches the light beautifully. Chain Necklace — Length & Weight Guide. 18" 1.78 GM 1.2 mm. 16" 1.65 GM 1.2 mm. 20" 2.02 GM 1.2 mm. 14" 1.49 GM 1.2 mm. 24" 2.29 GM 1.2 mm.