We report a novel nonequilibrium approach for direct laser writing diamond by melting amorphous polytetrafluoroethylene (PTFE:

(C

2

F

4

)

n

) in ambient conditions. The nanosecond laser pulses disintegrate PTFE

forming the undercooled molten carbon. This undercooled molten carbon regrows into diamond during the ultrafast melt quenching process

which lasts for ∼100 ns. HRTEM imaging

SAED

Raman

and EEL spectroscopy investigations confirm the first-order phase transformation of PTFE into single-crystalline

110

oriented diamond

which is associated with ultrafast unseeded crystallization. Our experimental findings open up a new pathway for the selective conversion of organic polymers into nano and microdiamonds and thin films with laser-writing.