Archaeological investigation of Olmec basalt workshops suggest that the colossal heads were first roughly shaped using direct percussion to chip away both large and small flakes of stone . The sculpture was then refined by retouching the surface using hammerstones , which were generally rounded cobbles that could be of the same basalt as the monument itself , although this was not always the case . Abrasives were found in association with workshops at San Lorenzo , indicating their use in the finishing of fine detail . Olmec colossal heads were fashioned as in @-@ the @-@ round monuments with varying levels of relief on the same work ; they tended to feature higher relief on the face and lower relief on the earspools and headdresses . Monument 20 at San Lorenzo is an extensively damaged throne with a figure emerging from a niche . Its sides were broken away and it was dragged to another location before being abandoned . It is possible that this damage was caused by the initial stages of re @-@ carving the monument into a colossal head but that the work was never completed .