The automated processing of 3D models of cultural artifacts can be significantly improved with formally defined high-level structured descriptors. Despite the large number of multimedia ontologies, however, the semantic enrichment of 3D models still has open issues. Many 3D ontologies are semi-structured only, cover a very narrow knowledge domain, do not provide comprehensive coverage for geometric primitives, or do not exploit the full expressivity of the implementation language. This paper presents the first attempt to transform the entire XML Schema-based vocabulary of the latest version of the X3D ISO standard (ISO/IEC 19775, 19776, and 19777) to OWL 2, complemented by fundamental concepts and roles of the 3D modeling industry not covered by X3D. The result of this effort is the most comprehensive formally grounded 3D multimedia ontology to date with standard alignment, which can be used for the representation, annotation, and efficient indexing and retrieval of 3D models.