The Hunter-Schreger bands are optical phenomena observed in tooth enamel when viewed under polarized light. These bands appear as alternating light and dark stripes and are crucial in understanding the enamel's microstructure.

The primary cause of the Hunter-Schreger bands is the alternating orientation of enamel rods. Enamel rods (or prisms) are the basic structural units of enamel, and they are arranged in layers with differing directions. When polarized light passes through enamel, it is refracted differently depending on the orientation of these rods. This results in the characteristic alternating light and dark bands known as Hunter-Schreger bands.

To clarify why other options are incorrect:
- Variable mineral content in enamel does not produce these bands; mineral content is relatively consistent throughout enamel.
- Gradual change in dentin tubule direction relates to dentin, not enamel, and therefore does not affect Hunter-Schreger bands.
- Presence of incremental lines of Retzius represents growth lines in enamel but does not cause the optical banding pattern seen with polarized light.

In summary, the alternating orientation of enamel rods is the structural feature responsible for the appearance of Hunter-Schreger bands under polarized light.

Reference: Ten Cate's Oral Histology, 9th Edition, Chapter 3 – Enamel Structure, Page 38