Which mineral inclusion is known to enhance the star effect in star sapphires?

Rutile is the correct answer because it consists of needle-like inclusions that can effectively enhance the star effect in star sapphires. This optical phenomenon occurs when light reflects off these aligned rutile inclusions within the stone, creating a star-shaped pattern that moves across the surface when the gemstone is rotated. The presence of rutile is crucial for achieving the desired visual effect, as its unique crystal structure and orientation allow for optimal light diffraction.

In contrast, the other minerals mentioned do not contribute to this star phenomenon in the same way. Hematite is generally opaque and does not have the same optical properties necessary for creating a star effect. Fluorite, while colorful and interesting, does not contribute to the unique light reflection needed for asterism. Kaolinite is a clay mineral that does not possess the characteristics required to create the star effect at all. Thus, rutile remains the key inclusion for enhancing the star effect in star sapphires.