Which of the following materials is recognized as a form of quartz used in dentistry?

The material that is recognized as a form of quartz used in dentistry is pumice. Pumice is a volcanic rock that is primarily composed of silica, and its structure includes a composition of quartz. This makes it particularly useful in dental procedures, especially in coronal polishing, where its mild abrasive properties help to remove stains and plaque from tooth surfaces effectively.

While chalk can also be used in dental polishing, it is not a form of quartz but rather a soft limestone consisting mainly of calcium carbonate. Leatherette is a man-made material and has no relevance in dental abrasive applications. Sand, although it can contain quartz, is too coarse for dental polishing and isn't specifically recognized in dental uses. Therefore, pumice stands out as the correct answer due to its specific composition and application in the dental field.