Educational researchers and teachers are well aware that misconceptions--erroneous ideas that differ from the scientifically accepted ones--are very common amongst students. Daily experiences, creative and perceptive thinking and science textbooks give rise to students' misconceptions which lead them to draw erroneous conclusions that become strongly attached to their views and somehow affect subsequent learning. The main scope of this study was to understand what students consider a mineral to be and why. Therefore, the goals were (1) to identify eleventh-grade students' misconceptions about the mineral concept; (2) to understand which variables (gender, parents' education level and attitude towards science) influenced students' conceptions; and (3) to create teaching tools for the prevention of misconceptions. In order to achieve these goals, a diagnostic instrument (DI), constituted of a two-tier diagnostic test and a Science Attitude Questionnaire, was developed to be used with a sample of 89 twelfth-grade students from five schools located in central Portugal. As far as we know, this is the first DI developed for the analysis of misconceptions about the mineral concept. Data analysis allows us to conclude that students had serious difficulties in understanding the mineral concept, having easily formed misconceptions. The variables gender and parents' education level influence certain students' conceptions. This study provides a valuable basis for reflection on teaching and learning strategies, especially on this particular theme. (Contains 7 tables and 1 figure.)