Final answer:
Light reflected at 62.5° from a gemstone in a ring is completely polarized. The gem cannot be a diamond.
Step-by-step explanation:
The phenomenon described in the question is known as polarization by reflection. When light is reflected at a specific angle from a surface, the reflected light becomes completely polarized. In the case of the gemstone in the ring, if the reflected light is completely polarized, it indicates that the gem is not a diamond. Diamonds do not exhibit complete polarization of light at any angle of reflection.