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The platy nature of sand-sized quartz particles reported by Wellendorf & Krinsley (1980) is corroborated by the results of an exercise given in my optical crystallography course. In order to bridge the gap between the immersion oils technique and thin-section work, the exercise asks the students to determine the sieve mesh-size interval of crushed quartz particles that best corresponds to the standard thin-section thickness of 0·03 mm, by determining the sieve interval which has the largest percentage of straw-yellow interference colours. Consistently, this interval has been found to be the −140 to +170 mesh interval (0·10-0·09 mm).