Which of the following describes surface characteristics in pearls?

Blemishes or irregularities confined to the pearl's surface accurately defines the surface characteristics of pearls. In the context of pearls, surface characteristics refer to the visible marks, spots, and irregularities that can affect the pearl's overall appearance and quality. These surface blemishes can arise from various factors, such as the conditions during the pearl's formation or the handling and care of the pearls over time. Identifying and assessing these imperfections is crucial for determining the value and grade of a pearl, as they can significantly influence its aesthetic appeal.

Other options focus on different aspects of pearls. For instance, structural composition relates to the material makeup of the pearl itself, including the nacre layers that form its body, rather than the surface features. Internal features pertain to what lies within the pearl, such as inclusions or irregularities not visible from the surface, which do not fall under the category of surface characteristics. Size and shape, while important attributes of pearls, describe the physical dimensions and contours rather than the specific details of surface quality. Thus, the focus on surface blemishes embodies the essential concept of what defines surface characteristics in pearls.