What characteristic is most commonly observed in sapphires?

The correct choice is color zoning, which refers to the uneven distribution of color in a sapphire. Sapphires are known for their beautiful hues, but it’s not uncommon for natural specimens to exhibit areas of varying color intensity or saturation within the same stone. This characteristic can provide insight into the gem’s formation and the environmental conditions present during its development.

Color zoning is often an important factor in gemology and can significantly influence a stone's value. Gemologists and jewelers consider the presence and intensity of color zoning when evaluating and pricing sapphires. In contrast, while clarity, size, and cut can also affect a sapphire's overall quality and marketability, they do not define the most commonly observed characteristic in sapphires. Color zoning is particularly distinctive in this gemstone, making it a more noteworthy feature.