A gnat's compound eye has multiple lenses called these, like the surfaces of a cut gem
[DOC] [TLE] Summer 2006 Gems & Gemology - GIA [PAR] To obtain a Japanese translation of Gems & Gemology, contact GIA Japan, 
Okachimachi Cy Bldg., 5-15-14 ... Diamond cut, the most complex of the 4 Cs, 
has.