What materials will hold microwaved heat the longest?

I’m cold. Really cold. I live in Ohio but used to live in California. I want to knit pocket warmer bean bags and fill them with something, then put them in the microwave awhile and heat those bitches up! What’s the most awesome thing I can put in those bean bags? I’ve seen instructions for these things that use rice, or beans. Sucko! I’m looking for suggestions for maybe space-age ceramic? Better yet, some other stuffing I can get a Michaels, but something that will stay hot forever! Okay, maybe not forever, but a really long time. Any cold physicists on Fluther?