Diamond coatings were deposited on diamond-polished molybdenum substrates from a premixed oxyacetylene flame for a long time (up to 4 h) at substrate temperatures between 700 and 950 degrees C, acetylene-to-oxygen ratios 1.02-1.07 and total flow rates between 230 and 310 standard L h(-1). The coatings contain, in addition to the densely populated octahedral crystals making a continuous layer of determined thickness, a number of individual large cuboctahedral crystals sticking out far above the layer. A large cuboctahedral crystal is formed from an octahedral one when the latter reaches a certain height at which its temperature becomes sufficiently high for the octahedron-to-cuboctahedron conversion to take place. This conversion was found to occur by a flattening of the octahedron pyramid tip whereby a {1 0 0} face perpendicular to the growth direction is formed. Both the height of the crystal and size of the {1 0 0} face increase upon further deposition, reaching up to 230 mu m above the octahedral crystals layer and up to about 200 mu m, respectively. The large crystals have smooth {1 0 0} faces, but otherwise often have an irregular shape which may be due to high temperature favouring deposition of non-diamond carbon.