Asterism in star sapphire is caused by which mineral?

Asterism in star sapphire is caused by the presence of specific inclusions, primarily composed of rutile, which is a mineral made of titanium dioxide, typically found within a host of corundum (the mineral family that includes sapphires). While hematite is not responsible for asterism, it is a mineral often discussed in the context of coloration in certain gemstones, including some other varieties of corundum.

The phenomenon of asterism occurs when light reflects off these inclusions, creating a star-like pattern on the surface of the gem. This distinctive optical effect is particularly celebrated in star sapphires, which exhibit a six-rayed star when viewed under direct light.

The role of quartz is significant in discussing a variety of gemstones, but it is mainly a different mineral altogether and does not contribute to the star effect in sapphire. Magnetite and graphite may have their place in discussions of gemstone properties but do not play a part in generating asterism in sapphires. The correct understanding of how asterism arises is crucial for gemology, particularly when identifying star sapphires within the extensive array of colored gemstones.