Optical relief (usually noted as simply relief) is a concept in optical mineralogy which refers to the degree in which mineral grains stand out from the mounting medium, usually either oil with a known refractive index or Canada Balsam. Relief is an important part of the Becke line test.
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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".