These flat-wire octagonal titanium key rings have a nice look and feel, and the octagon Swarovski Strass crystal continues the geometry. (If you it's crazy to put a crystal in a pocket with keys ... See links below for other design possibilities!)
Pick the largest (thickest) jump ring that will fit through your chain link, and add the chain to the key ring. For best way to open and close loops, see Jump Ring Hint in Jewelry Basics 101 (PDF)
Pick the thickest jump ring that will fit through the crystal, and add to other end of chain.
Carefully inspect your jump rings to make sure they are securely closed. Key rings generally take a lot of abuse.
1 each #41-919-0225 Titanium 25mm Octagon Flat Wire Split Ring
1-2" of your favorite gunmetal chain. I used #40-090-45-9 Gunmetal Drawn Flat Cable Chain, 3.7mm, because I had a scrap on my workbench. However, I think #40-090-50-9 Gunmetal Oval Square Wire Cable Chain, 5.8mm would have looked better with this key ring.
2 Pairs of Flat-Nose Pliers (chain-nose and bent-chain nose are nice options)