What is used to seal around glass in fireplace doors?
The usual material is fiberglass rope or woven fiberglass tape. 

For sealing glass, the rope fits in a groove, the tape is used between a flat surface and the glass with additional pieces under the clips to cushion their grip on the window glass.

The dark stuff is impregnated with graphite to prevent it sticking to the opposite surface (door seal). 

Rutland is one brand with a broad selection of sizes and types.