The understandings of properties that children develop in the context of rock and mineral identification point to the challenges of observing in a manner authentic to the discipline of geology. The notebook entries of 34 third graders written during the enactment of a commercial unit on earth materials were investigated for their descriptions of rocks and minerals, and the extent to which these reflect understandings of properties in both generic and discipline-specific senses. While children were successful in observing minute details of rock and mineral samples, there were few opportunities within the curriculum for students to differentiate between geologically relevant and irrelevant properties, and there were no connections drawn between deeper geological concepts and these observations. In addition, the demands placed on young learners to write their observations in the form of descriptions raise issues of supporting students in their learning of authentic scientific writing. There are challenges faced in representing geological observation authentically, and moving children beyond simple description activities into more complex explanations. Suggestions for improvements that attend to issues of the discipline as well as the elementary context are provided. © 2005 Wiley Periodicals, Inc. Sci Ed89:276–295, 2005