What is asterism in gemstones?

Asterism in gemstones refers to the optical phenomenon that occurs when light reflects off inclusions within the stone and creates a star-like pattern on the surface. This effect is typically observed in cabochon-cut gemstones, where the stone's surface is highly polished and smooth.

The presence of fine intersecting needle-like inclusions, such as rutile or hematite, within the gem is crucial for producing asterism. When light enters the stone, it reflects off these inclusions and bounces out in a way that forms a star shape, which is most easily seen when viewed under a direct light source. The classic examples of gemstones that exhibit asterism include star sapphires and star rubies, where the star appears to move as the viewing angle changes.

While other options present concepts related to gemstones, they do not align with the definition of asterism. For instance, inclusions are part of asterism but focusing on their crossing and the resulting star pattern is what defines asterism specifically, rather than just a general presence of inclusions or characteristics of other decorative techniques or optical effects in different gemstones.