Crystal nucleation takes place along the grain boundaries of other crystalline phases during many geological and synthetic processes, but predicting exactly where and how a new phase nucleates has remained a challenge. In this talk, I present a new model showing that crystal nucleation tends to follow specific geometric rules, where new phases align themselves as to maximize atomic bonding with the surrounding material—a process known as epitaxy. By analyzing a wide range of natural mineral pairs, I demonstrate that in metamorphic crust this epitaxial nucleation is not the exception but the rule. This discovery provides a powerful new way to predict how reacting crystalline materials evolve, with implications for everything from crustal rheology and the fabric in Earth's deep interior to advanced manufacturing.