sentence: For example , carbon can be found as diamond , which has a tetrahedral structure around each carbon atom ; graphite , which has layers of carbon atoms with a hexagonal structure stacked on top of each other ; graphene , which is a single layer of graphite that is very strong ; fullerenes , which have nearly spherical shapes ; and carbon nanotubes , which are tubes with a hexagonal structure ( even these may differ from each other in electrical properties ) .
question: how is something structured _ _ _ ?
answers: stacked on top of each other with a hexagonal structure