Optical relief is a concept in optical mineralogy

Optical mineralogy

Optical mineralogy is the study of minerals and rocks by measuring their optical properties. Most commonly, rock and mineral samples are prepared as thin sections or grain mounts for study in the laboratory with a petrographic microscope...

 which refers to the degree in which mineral grains stand out from the mounting medium, usually either oil with a known refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 or Canada Balsam

Canada balsam

Canada balsam, also called Canada turpentine or balsam of fir, is a turpentine which is made from the resin of the balsam fir tree of boreal North America...

. Relief is an important part of the Becke line test

Becke line test

The Becke Line Test is a technique in optical mineralogy that helps determine the relative refractive index of two materials. It is done by lowering the stage of the petrographic microscope and observing which direction the light appears to 'move' toward. This movement will always go into the...

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Magnitude

Minerals that stand out significantly (have a difference in refractive index of .12 or more) have high or strong relief, and will have very sharp boundaries between itself and the material it is next to. Intermediate is .04 to .12, and low or weak is less than .04. Low relief materials have boundaries that are hard to distinguish from each other.

Polarity

Positive relief refers to a mineral that stands out higher than the medium, and negative relief is a mineral that appears to "sink in".