Emerald mineralization is associated with ultramafic
to mafic rocks (metamorphosed into amphibolites)
within the Santa Monica Shear Zone. These rocks are
located at relatively higher altitude (>400 m) in the
resistant granite gneiss basement. Adjacent to the
amphibolites is a succession of talc, talc-amphibole,
and biotite±amphibole schists, with a sharp contact
between biotite schist and granitic gneiss (figure 4). The
biotite schist layers range from 0.2 m to more than 3
m thick, and dip northwest rather steeply at 38º–56°.
Small recrystallized pegmatitic bodies are present in
the schist, near the contact with the granitic gneiss.
Emeralds occur in and around those bodies, particularly
within the biotite schist.