University of Exeter physicists specialising in colour and light have done experiments exploring the origin of the scarab beetles' striking metallic golden appearance, showing that the golden beetles have a unique 'optical signature'. The structure of the beetle and its armour uniquely manipulates the way the light is reflected so that it looks like pure gold.

https://m.phys.org/news/2017-06-secret-jewel-scarab-beetles-pure.html