Heat up the hot glue gun. Slowly dispense the glue around the inside of the gem mold. Go slowly and tap the mold on a hard surface every few minutes to settle the glue.
Wait a few minutes until the top is still tacky, but not fluid. Insert a plastic stir stick into the middle of the gem shape. Use something like a clothespin to hold it straight.
After 30 minutes, the top should feel completely dry and hardened. Carefully remove from the mold.
Paint at least two coats of paint on the gem portion of your DIY drink stirrers and let dry.
Notes
For best results, spray a sealer over the paint to reduce the potential for chipping over time.