Which of the following is NOT considered in the estimation of gingival index?

The estimation of the gingival index is a clinical assessment that evaluates the health of the gingiva by examining specific characteristics. The main criteria taken into account include the color and consistency of the gingiva, bleeding on probing, and swelling of the gingiva. Each of these factors helps dental professionals assess inflammation and the presence of any periodontal disease.

Nasmyth's membrane, however, is not a criterion considered in the estimation of the gingival index. It refers to a thin membrane that can cover the newly erupted teeth, particularly in children, and is made up of epithelial cells and dental cuticle. While it can influence oral hygiene procedures and the presence of plaque, it does not directly relate to the inflammation or health of the gingiva as evaluated in the gingival index assessments.

Therefore, identifying Nasmyth's membrane as not being part of the gingival index estimation is accurate, as it does not pertain to the clinical measurement of gingival health.