Mud logging, also known as hydrocarbon well logging, is the creation of a detailed record of a borehole by examining the bits of rock or sediment brought to the surface by the circulating drilling medium (most commonly mud). Mud logging microscopes are used to classify different rock types and pores of the cuttings examined for oil traces and their ability to hold oil and gas. Pseudo-darkfield illumination is used to bring out features of a specimen not normally seen under brightfield light.