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