We present broadband observations and analysis of Swift  gamma-ray burst ( GRB ) 120119A .
Our early-time afterglow detections began under 15 s after the burst in the host frame ( redshift z = 1.73 ) , and they yield constraints on the burst energetics and local environment .
Late-time afterglow observations of the burst show evidence for a moderate column of dust ( A _ { V } \approx 1.1 mag ) similar to , but statistically distinct from , dust seen along Small Magellanic Cloud sightlines .
Deep late-time observations reveal a dusty , rapidly star-forming host galaxy .
Most notably , our early-time observations exhibit a significant red-to-blue colour change in the first \sim 200 s after the trigger at levels heretofore unseen in GRB afterglows .
This colour change , which is coincident with the final phases of the prompt emission , is a hallmark prediction of the photodestruction of dust in GRB afterglows .
We test whether dust-destruction signatures are significantly distinct from other sources of colour change , namely a change in the intrinsic spectral index \beta .
We find that a time-varying power-law spectrum alone can not adequately describe the observed colour change , and allowing for dust destruction ( via a time-varying A _ { V } ) significantly improves the fit .
While not definitively ruling out other possibilities , this event provides the best support yet for the direct detection of dust destruction in the local environment of a GRB .