Blue is the natural color of glaucophane in plane-polarized light. You can
see that individual crystals vary from blue to lavender to colorless
depending on their orientation. The crystals have a zillion orientations in
the three-dimensional world of the rock, but keep in mind that the thin
section is only a two-dimensional slice. Plane polarized light interacts in
specific ways with grains that are positioned differently to the light path.
The slight color variations that result serve to identify glaucophane
because only two other very rare minerals show comparable color. Rocks
that contain abundant glaucophane are called blueschists.
Plane polarized light