Graphite can withstand a ton of heat. It is also very easy for a glassblower to shape graphite to a more usable tool compared to a hard metal like steel.
Basically carbon, it is the most stable form of carbon under standard conditions.
Graphite by a Glassblower – This will be found in any blowers arsenal. Usually it will be in flat plates that a blower will roll their glass on, or it will be in the form of a rod. The graphite can withstand shit tons of heat and can also be put in water without damage. Its easy to grind or carve for a simple mold.