As much as I would love to think that MM's headband is a diamond riviere, I suspect it is just a rhinestone/paste headband. My reasoning is that it does not seem to be MM's style to go with such real "bling" during the day. She is normally a conservative dresser and doing a headband with diamonds just seems out of character for her. Granted she has a lovely aquamarine brooch at the neckline of her dress, but it seems appropriate for daytime. A headband of diamonds during a christening service would be way to over the top, especially for her.
On an off topic, it is great to see that she was able to make the trip and sit thru the service with her ongoing back issues. As someone who has lived with someone who has chronic back issues, you never know literally from one moment to the next when you will be feeling perfect and when you are flat on your back in agony. Chronic back pain can come and go almost at the blink of an eye. And if you have a disc that bulges into a nerve, it can be excruciating until it passes.