The Mineralogy Concept Inventory (MCI) is a statistically validated 18-question assessment that can be used to measure learning gains in introductory mineralogy courses. Development of the MCI was an iterative process involving expert consultation, student interviews, assessment deployment, and statistical analysis. Experts at the two universities at which the MCI was developed were consulted in detail to identify key concepts in their mineralogy courses. Additional experts who have recently taught introductory mineralogy courses in postsecondary institutions were consulted via an online survey. These experts (n = 75) indicated that, on average, 87% of their total course content is included within the 7 key concepts targeted by the MCI. Think-aloud interviews with students were conducted to generate response options written in student language, providing the greatest chance that students will understand the questions as intended. The validity and reliability of the questions on the MCI were evaluated using Rasch analysis to ensure the assessment includes questions that range from high to low difficulty, that it can distinguish between low- and high-performing students, and that the test is not biased toward any subset of the population.