sorry but the seat belt would have been burned too if the plane was set on fire. (the one he pulled off to make a tourniquet)
(at least I would THINK)
hmmm... in "The Cost of Living," Eko goes back to the burned plane, and there are Virgin Mary statues still intact inside the plane, along with some other junk. Possibly the makeshift-tourniquet was unburned as well? Also, the rock that Locke rests against is just where Eko & Locke moved it in that same episode...though it could have been there before he lit up the plane. Maybe there'll be a definitive answer in an upcoming episode...