This is a fine introductory activity for a geology unit. You will need several rock samples. They may be rotated among the students, but to allow for different pacing, you will need at least a couple per student.
In this activity, students are expected to sketch and describe four separate samples. An emphasis of the activity is to be able to describe the samples in scientific terms, using quantities and specific adjectives. The sketches are meant to show specific features of the samples and not