Final answer:
The 6-pointed stars seen in rubies and sapphires are due to the reflection of light off internal rutile inclusions, causing the asterism effect, which is separate from the coloration caused by Cr3+ impurities.
Step-by-step explanation:
The 6-pointed stars seen in certain rubies and sapphires are due to a phenomenon called asterism. This effect occurs when light is reflected off of the internal structures within the gemstone, typically tiny needle-like inclusions of rutile (TiO2). When light hits these inclusions, it is reflected and scattered in multiple directions, leading to the appearance of a star-like pattern on the surface of the stone. The alignment and intersection of these inclusions are what primarily cause the phenomenon. It is interesting to note that this effect is not directly related to the presence of Cr3+ impurities in the lattice of the gemstones, which are the cause for the vibrant red and green hues of rubies and emeralds, respectively.