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 the general essence of them.
As students are sketching, remind them to draw just what they see. This is a difficult concept for some students. Explain that if they draw a large bump on a rock, it should be a bump that is present in the same place and about the same size on their sample.
If you have scales and rulers available, have student include weights and dimensions in their descriptions. Explain that they should avoid relative adjectives like large and small, as well as vague terms like pretty or heavy. Some terms that would be acceptable are shiny, speckled, rough, smooth, or layered. Colors are an important description. In some cases similes are acceptable if the sample is being compared to something specific rather than subjective. For example, it would be fine to say a rock sample sparkles like crumpled aluminum foil but not to say it sparkles like a friend's smile.
After completing their sketches and descriptions, students are asked to create a movement phrase (use the word dance, if you think the kids won't resist) for one of their samples. Then each student presents their movement to the class, and their classmates try to figure out which of their samples they are presenting.
I had the idea to try adding movement to this activity, but I wasn't sure how it would go. It was an excellent addition. Some students made self-deprecating remarks about their dances, but everyone tried it. Incorporating movement into this activity is a way to engage kinesthetic learners, and even for those students who are not as comfortable adding this element, it engages a different part of their brains in remembering key concepts.