Abstract: The present invention relates to a process for increasing the dimensions of quartz crystals and of quartz-isomorphic materials, which can be produced by hydrothermal growth from seed, in which a seed obtained by assembly of several crystalline plates is used, wherein there are assembled at least two plates of strictly identical lengths and/or widths and of similar thicknesses, cut in at least one crystal and having, on at least three faces crystalline orientations, the deviations from which remain, between the plates, smaller than about 10" of arc, and wherein said plates are brought together on a support by guidance elements, thus ensuring their alignment on the aforementioned three faces while maintaining crystalline orientation deviations smaller than about 10" of arc, one face per plate being left free for growth of the assembled seeds, leading to the production of a single crystal exhibiting at least one dimension greater than that corresponding to the original crystalline plate.