A gnat's compound eye has multiple lenses called these, like the surfaces of a cut gem
Sometimes a 58th facet, called a culet is 
cut on the bottom of the stone to... [DOC] [TLE] Summer 1999 Gems & Gemology - GIA [PAR] Color separations for Gems & Gemology are by Pacific Color, Carlsbad, ..... of 
such surface-reaching features (hereafter called ...