Petrography Testing Petrographic testing is the use of microscopes to examine samples of rock or concrete to determine their mineralogical and chemical characteristics. Samples for petrographic examination can be taken from lump samples or cores. They are impregnated with resin. Suitable surfaces (either polished or thin sections) are prepared by sawing, lapping and polishing. The samples are examined through a petrological (geological polarising) microscope, using either reflected or transmitted light. The lighting may be normal, ultra-violet or polarised.