An uncoated but textured head made out of 10mil Mylar (the head has an etched surface). If you use sticks at all (rather than brushes) don't expect the texture to survive - mine lost the texture very quickly (though I do use Nylon-tipped sticks and perhaps the texture would be more durable with wood tips). But the texture on a standard coated G1 is more resilient.
What you get is the very open sound of an uncoated head. And these heads do sound great.