How many rays are typically seen in a star corundum?

In star corundum, the phenomenon known as asterism is responsible for the appearance of star-shaped patterns on the surface of the gemstone. This effect is caused by the presence of needle-like inclusions, typically rutile, which are oriented in specific directions within the crystal structure.

The most common type of asterism seen in star corundum displays a six-ray star. This is due to the arrangement of the rutile inclusions, which intersect at angles that create a six-pointed star effect when viewed under a light source. The light interacts with these inclusions, producing the star's visibility as it moves across the surface of the stone.

While four-ray and eight-ray stars can occur in other gemstones or with varying conditions in some corundum, the conventional six-ray star is the most recognized and is what is typically referred to when discussing star corundum. The twelve-ray star can occur but is much rarer and is not commonly associated with corundum. Thus, the visibility of the six rays in star corundum is the standard case, making it the correct answer.