What is the white stuff on the rubber gasket?

I’ve been caning for a bit now and I don’t know what this white stuff under the lid is so I thought I would ask here.

For context I’m up in Canada and this is a gem jar with a brand new viceroy rubber ring and a glass lid. I canned these turnips a couple of weeks a go. I followed the tested recipe from the Ball complete book. For the lid instructions I follow Healthy Canning’s advice. I did add vitamin C to prevent discoloration when I was preparing and I have hard water.

I hope someone here knows what this is under the lid . TIA !