These popular ring stitch markers from Cocoknits are made of nylon coated steel. You get 10 each of 6 bright fun colored stitch markers so you can easily mark not only the beginning of your rounds, but changes in stitch patterns, increase and decrease placement, and more. They're small and smooth and come in a cute little round box that won't fall open in your knitting bag! Stitch markers fit needles from US size 8 (5.0mm) up to US size 13 (9.0 mm)
For needles US size 7 (4.5 mm) and smaller, try the mini version here.