Conventionally, crystals are infinitely repeating lattices of atoms or molecules, in which every lattice site is identical to every other. Clusters of clusters are an alternative to strict crystalline arrangements, and could form an entirely new family of condensed matter. Such a structure has now been created: hierarchically clustered icosahedral particles of boron sub-oxide can be grown to sizes several micrometres across. As well as being of theoretical interest, this substance is very hard, and could be a useful engineering material.
