Is the facial hair of Tolkien's dwarves unshaveable?

Per The History of Middle Earth Vol. 11

The Naugrim were ever, as they still remain, short and squat in stature; they were deep-breasted, strong in the arm, and stout in the leg, and their beards were long. Indeed this strangeness they have that no Man nor Elf has ever seen a beardless Dwarf - unless he were shaven in mockery, and would then be more like to die of shame than of many other hurts that to us would seem more deadly.

It's not clear how this fact is known, but that a dwarf can be shaved (if under duress) is evident.

---

Their beards can also be burned by dragonfire.

"The few of us [dwarves] that were well outside sat and wept in hiding, and cursed Smaug; and there we were unexpectedly joined by my father and my grandfather with singed beards. They looked very grim but they said very little.

_{^{The Hobbit, or There and Back Again}}