(physics) visible streaks produced in a transparent medium as a result of variations in the medium's density leading to variations in refractive index. They can be recorded by flash photography (schlieren photography)
streaks or platelike masses of mineral in a rock mass, that differ in texture or composition from the main mass
Irregular dark or light streaks in plutonic igneous rock. Schlieren have the same general mineral composition as the rocks in which they are found, but they are usually slightly darker or lighter than the rest of the rock because of differences in the ratios of the mineral types they include. They are typically a few centimeters to tens of meters long and can form in various ways, including by sorting of minerals during magma flow and through the gravitational settling of minerals during magma cooling and solidification.
Regions of a transparent medium, as of a flowing gas, that are visible as light or dark areas because their densities are different from that of the bulk of the medium.