ABIBLO GEMOLOGY - What is Aberration?

What is Aberration? 
Aberration (ab″er-ae′shun) is  the failure of a lens or mirror to bring the light rays to the same focus. When aberration is due to the form of the lens or mirror it is called spherical aberration. When due to the different refrangibility of light of different colors, it is called chromatic aberration. When present in magnifiers it often causes inaccurate decisions as to flawlessness or color of gems.