Which feature differentiates star sapphires from regular sapphires?

Star sapphires are distinguished from regular sapphires primarily by the presence of a star-like phenomenon, known as asterism. This optical effect occurs due to the inclusion of needle-like formations of rutile, which create a star shape that appears to move across the surface of the stone when viewed under direct lighting. The star can appear as a single line or as a more complex pattern, depending on the arrangement of the rutile needles.

The brilliance of color, clarity, and hardness are important characteristics of all sapphires but do not specifically differentiate star sapphires from other varieties. While star sapphires can have striking colors and varying clarity levels, what truly sets them apart is this unique star effect. Thus, the defining feature of a star sapphire is indeed the presence of the star-like phenomenon, which makes these gemstones particularly sought after and valued in the market.