The synthesis of diamond volume crystals is of particular interest due to the material's outstanding physical and mechanical properties. Various techniques are pursued by numerous research groups. In hot-filament CVD (HF-CVD) we found a new process parameter window where the growth of single diamond volume crystals can be stabilized. These parameters are far beyond growth conditions for HF-CVD diamond coating processes. Extremely low methane content in the feed gas along with high substrate temperatures allows single diamond nuclei of a certain minimum size to grow stable. Crystals up to 100 mum in diameter were successfully synthesized on ultrasonic pretreated substrates. They were characterized by SEM and Raman spectroscopy. A model for the observed exclusive growth of sufficiently large nuclei is presented. Based on well accepted diamond growth models, an advanced growth theory for the different morphologies and diamond qualities at the {100} and {111} faces of the crystals is postulated. The results give bases on which growing processes can be optimized and upscaled in order to evaluate the applicability of the method for single-crystal diamond growth. (C) 2004 Elsevier B.V. All rights reserved.