Gold clusters are gold compounds with a core of gold atoms and organic groups covalently bound to the surface gold atoms. An example is undecagold, Au11(P(C6H5)3)7, whose structure was solved by x-ray crystallography using 3-dimensional crystals. These differ from colloidal gold, which are suspensions of metal particles, usually formed by metal ion reduction; although the particles may be approximately the same size, they vary due to the statistical process of formation. Gold clusters are compounds with a definite formula, and should all be perfectly identical. However, it is known that there is a family of stable gold cluster compounds, such as Au6, Au11, Au13, AU55, Au67, etc. In a given preparation of gold clusters, there is usually some mixture of these, thus leading to some size variation. Methods such as gel filtration column chromatography and ultrafiltration can be used to separate most of these species, so that relatively pure preparations may be achieved.