This listing is for the very rare 'mirror glaze' or 'pikal' variety of Phalacrognathus muelleri (Rainbow Stag Beetles). The adults of this variety have a genetic mutation which results in the pronotum of their thorax being exceptionally shiny, rather than the naturally duller matte coating.
 
This gene was specifically selected by professional beetle breeders in Japan, it remains very rare in the UK. Additionally these adults appear to lay fewer eggs than other types, adding to their rarity.