Katara's necklace is a blue choker with a light blue, hand-crafted jewel bearing the waterbending symbol. But since I made this cosplay on a major budget, its just a plastic shrinky-dink.

Step 1: Step 1: What You'll Need

Materials

A circle template (mine was about 2 1/2")
Shrink plastic or #6 plastic
Scissors or an xacto knife
Blue marker

Step 2: Cutting the Plactic

Cut out the circle and fix the edges to be as round as possible

Step 3: Colour Your Plastic

You'll want to colour your plastic with some sort of blue marker (I used a Faber-Castell PITT artist pen) which is a waterproof Indian ink.

Step 4: Shrink the Plastic

Heat your oven or toaster oven to 350°, and place a rack in the lowest position. There's not really a specific amount of time you need to leave it in for, but I left mine in for a minute and a half. You can just watch it until it stops shrinking, and don't worry about the plastic curling up because it'll smooth on it's own.