Strong's G762 — ἄσβεστος (adjective)
asbestos (as'-bes-tos) adj.
1. not extinguished
2. (by implication) perpetual
[from G1 (as a negative particle) and a derivative of G4570]
KJV: not to be quenched, unquenchable 
Root(s): G1, G4570 
Hebrew root: σβέννυμι
Occurrences: 3