Have you ever tried this trick? You can put dry rocks on top your heater, radiator or wood stove and get them nice and warm (or burning raging hot)... then you can stick them either in your pockets, over your heart to warm your blood stream, or the most special place in your cold cold COLD BED! I like to put mine on the wood stove and get them so hot that I have to wrap them in a few layers of fabric, then I shove it under the covers in my bed and wait 5 or 10 minutes before getting in. This makes the transition to cold sheets in the winter 100 x's more bearable!! I snuggle up with those hot rocks and put them on my belly.
PS ::: Make sure your rocks are not wet and fresh from a stream, if they get too hot they could explode! Let them dry out first. :)