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[Fiebre Verde]

I recently came across this unusual Colombian crystal from the Chivor mining district.
We can observe a set of pyrite grains aligned along a plane perpendicular to the c-axis.
We can also observe a brown streak directly above each pyrite grain, which runs to the end of the crystal.

After a first growth step, inclusions (pyrite grains in this case) were attached to the crystal face.
In the next growth step, the presence of these inclusions prevented emerald material to be deposited directly above the pyrite grains, in the direction of growth. Consequently, elongated channels (or tubes) are formed along the crystal growth direction.
Such tubes may end within the crystal, but they may also reach the surface at the end of growth, as shown in this example.
In such tubes, there is normally fluid-filling, but there may be also other minerals.
The dark color of the channels is due to pyrite material dragged “upwards” during the last growth step, making the walls of the channels look brownish.

This feature is rarely seen in Colombian emeralds but is well described and understood for other minerals.

[James Catmur]

Interesting growth affect