Considerable F-for-OH substitution can occur in the structure of hambergite, Be 2 (BO 3 )(OH,F). The substitution results in a linear reduction of the indices of refraction. This relation can be used to estimate the F content of hambergite, subject to the constraint that samples may be compositionally heterogeneous. Previous X-ray-diffraction studies of five crystals of hambergite showed that increasing amounts of F for OH lead to a decrease in the a dimension and an expansion of the b dimension. The F content of hambergite may be readily estimated if the unit-cell dimensions are known. Although originally described from an alkaline syenitic pegmatite, the majority of hambergite occurrences are in complex Li-rich granitic pegmatites of the elbaite or transitional elbaite-lepidolite subtypes. Hambergite occurs as a comparatively early phase in massive pegmatites from several localities in the Czech Republic. More commonly, hambergite is a late-crystallizing phase produced in miarolitic pockets as the highly evolved magma approaches its solidus. The F content of hambergite from pegmatites appears to reflect the F activity of the medium (melt or fluid) from which it crystallizes. Hambergite found in miarolitic pockets is typically more homogeneous in composition and commonly has a lower F content than that found in a massive pegmatite, although F-rich examples from pegmatitic pockets also are observed. It is unclear whether the lower F content of hambergite from miarolitic pockets is strictly a result of crystallization in lower-F systems, or involves precipitation in equilibrium with exsolved aqueous fluids.