Model results and empirical data show that germanium is fractionated from silica during biogenic opal formation. In a two‐box model of the ocean, particulate Ge/Si can vary between the average ocean Ge/Si and the average ocean ratio times the Ge fractionation coefficient. The two‐box model requires a Ge fractionation coefficient near 0.48 to reproduce Si and Ge concentrations similar to the ocean. Analysis of paired water and opal samples yields a Ge fractionation coefficient of 0.51. As a consequence of this fractionation, the Ge/Si of opal can be an upwelling indicator. Small shifts of opal Ge/Si in adjacent centimeter‐scale lighter and darker layers in the Miocene Monterey Formation are consistent with other sedimentologic indicators of productivity and upwelling variations. Opal Ge/Si are lower in lighter layers and are consistent with interpretations that the lighter layers were formed by increased productivity in overlying w
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