They're a rubber composite over a steel frame. OldIron suggested them to me, and they're absolutely awesome. No leaks what so ever, and I was having troubles before...

Scrub down the heads and valve covers with brake cleaner and some scotch-brite. Blow off any residue. Use absolutely NO tack! Just put them in and tighten down the hold-downs. You're done. Best news is that they're re-usable, so you don't have to worry about them being destroyed when you yank your valve covers.

$31.95 at Summit Racing