Laser-induced damage in optical coatings is generally associated with micrometer-scale defects. A simple geometric model for nodule-shaped defects is commonly used to describe defects in optical coatings. No systematic study has been done, however, to prove the applicability of that model to an optical coating deposition process. Not all defects have a classic nodule geometry. The present study uses atomic force microscopy (AFM) and scanning electron microscopy to characterize the topography of coating defects in a HfO2/SiO2 multilayer mirror system. Focused ion-beam cross sectioning is then used to study the underlying defect structure. This work develops a model for defect shape such that the overall geometry of a coating defect, particularly the seed size and depth, can be inferred from nondestructive evaluation measurements such as AFM. The relative mechanical stabilities of nodular defects can be deduced based on the nodule’s geometry. Auger analysis showed that the seed material that causes nodular defects in HfO2/SiO2 multilayers is a hafnia oxide. Such characterization capabilities are needed for understanding the enhanced susceptibility of particular defects to laser damage and for developing improved techniques for depositing low-defect density coatings.