Students struggle with observing scientifically and connecting observations to scientific theory. This study investigates how students actually use observation in rock classification -- a classical practical task in science education. To describe the level of students' use of observation, data was collected by videotaping 19 small student groups (55 students aged 16-18) in Norway while they were classifying rocks. A modified version of the observation framework proposed by [Eberbach, C., & Crowley, K. (2009). From everyday to scientific observation: How children learn to observe the biologist's world. "Review of Educational Research," 79(1), 39-68] is used to analyse how students' notice features of rocks (noticing) and interpret the geological processes forming those features (expectations) at different levels: everyday, transitional or scientific. The findings showed that none of the student groups used everyday observation. Three student groups used observation at a transitional level, whereas twelve groups performed observation that can be described as transitional/scientific level. Four student groups used scientific observation. Based on the findings, an observation framework for rock classification is proposed. The challenges encountered by the students are discussed, thus providing ideas for how teachers can support students to use scientific observation in rock classification.