Abstract <p>Currently, more than 6000 minerals are known. Among them, a dozen mineral species may seem to be sufficient to form an initial picture of Earth’s dynamics. However, this picture is still incomplete as rare minerals give important insights into the complexity of our planet, in some cases revealing huge gaps in our understanding of geological processes. This awareness has promoted renewed interest in the study of rare minerals as clearly indicated by several studies published in the last decade and by the increasing number of new mineral species described since 2010. Rarity in mineralogy is due to several reasons, related to physical–chemical constraints as well as to sampling bias. Some case studies, based on the experience of the author, are discussed and emphasize the implications of this work. In particular, the role of mineralogical crystallography in deciphering the complex crystal chemistry of rare minerals is stressed because it allows the extraction of geological information from these unusual compounds. Although the study of rare minerals may seem to be of interest only to geoscientists, the accurate examination of mineral assemblages and the characterization of rare accessory minerals may highlight unexpected geochemical anomalies with potential implications on everyday life. Rare minerals can be considered as enhancing our understanding of Earth’s dynamics.</p> Graphic abstract <p></p>