Small rock and mineral specimens about one inch in size are usually adequate for student examination and identification. However, many rock units, identified as gneiss in the field, have bands that are thicker than one inch. If samples of these rock units are broken into one-inch pieces, many of them will be too small to exhibit the banding features of gneiss. This will confuse many students and cause others to incorrectly identify the rock.

Teachers can avoid these problems by collecting specimens that clearly display a banded structure. Teachers who purchase specimens must examine them carefully before they are presented to students.

After students have learned to identify gneiss and many other rock types, presenting specimens of gneiss that do not exhibit banding can be a challenging way to have students: A) consider possibilities that are not obvious, and, B) realize that a single rock specimen may not adequately represent a rock unit.