Why do reputable gem laboratories refrain from grading mounted diamonds?

Reputable gem laboratories refrain from grading mounted diamonds mainly because mountings can obscure essential details regarding the diamond's clarity and cut. When a diamond is set in a piece of jewelry, particularly in intricate settings, certain characteristics that are crucial for a proper evaluation may be hidden from view. For example, inclusions, blemishes, and the overall symmetry or proportions of the diamond's cut can be partially or completely concealed by the mounting. This lack of visibility makes it difficult for gemologists to assess the diamond's quality accurately and provide a reliable grade.

The other options do not capture the primary reason for this practice. While clarity may sometimes appear enhanced in a mounting, this is not the driving factor behind the decision to avoid grading mounted diamonds. Furthermore, grading is not inherently more expensive for mounted diamonds, nor are mounted diamonds generally of lower quality; many are quite exquisite. The crucial issue remains that the integrity of the assessment cannot be maintained when critical details are obstructed, which is why laboratories emphasize grading diamonds in their unmounted state.