Computational mineralogy is fast becoming the most effective and quantitatively accurate method for successfully determining structures, properties and processes at the extreme pressure and temperature conditions that exist within the Earth's deep interior. It is now possible to simulate complex mineral phases using a variety of theoretical computational techniques that probe the microscopic nature of matter at both the atomic and sub-atomic levels. This introductory guide is for geoscientists as well as researchers performing measurements and experiments in a lab, those seeking to identify minerals remotely or in the field, and those seeking specific numerical values of particular physical properties. Written in a user- and property-oriented way, and illustrated with calculation examples for different mineral properties, it explains how property values are produced, how to tell if they are meaningful or not, and how they can be used alongside experimental results to unlock the secrets of the Earth.