The present work is the first comprehensive study of mineralogy of the worlds second largest mercury mine Idria. In the first chapter the readers are acquainted with the history of mining and its relation to mineralogy. Mineralogy is explained in terms of geological processes that were active during the formation of the ore deposit. Among these, hydrocarbons present in the host rocks play a very important role during crystallization. The central part of the book is dedicated to the main mineral of the ore deposit, cinnabar. It occurs in a variety of crystal forms, of which the most special are lateral interpenetration twins. The book is written for a broad readership, and will be interesting for geologists, mineralogist and crystallographers, as well as for those interested in the history of mineral collecting in Idria.