Thumbtack

Sample: A transparent thumbtack viewed through crossed polarizing filters.
Source: Thumbtack: the floor of my office, polarizing filters: American Science and Surplus.

Polarized light is affected by its travel through a material. In crystals, this can serve as a way to ascertain their chemical identity (different chemicals rotate the polarity of the light in different amounts). In polymers and glasses, this can let you see the internal stress on the object, as different amounts of stress produce different bands of color.



Thumbtack, 10x.



Thumbtack, 60x.



Thumbtack, 200x.