Finished with a polished surface that catches the light beautifully. Chain Necklace — Length & Weight Guide. 22" 8.25 GM 3.5 mm. 16" 6.09 GM 3.5 mm. 18" 6.79 GM 3.5 mm. 20" 7.5 GM 3.5 mm. 24" 8.9 GM 3.5 mm.