The man was filmed by CCTV in Coopers Jewellery in Barnstaple in Devon on Thursday.
Owner David Bruce said he "chased" the suspect from his shop with a baseball bat following the attempted raid.
Devon and Cornwall Police said they believed the would-be robber's "weapon was some kind of blank firing firearm".
Det Insp Praveen Naidoo said: "The offender may well have received injuries as a result of this incident and it is likely that someone in the community knows who is responsible.
"Whilst the coat appears darker in the images, witnesses describe it as a light sandy colour."
Anyone who has found the mask, which Mr Bruce described as a "full latex" one, is asked to contact police.
Footage of a masked man wearing sunglasses and wielding a gun has been released by police after an attempted armed robbery at a jewellers.