Nehaveigur

Some chemical substances assemble in different crystal structures without changing their composition. Those alternative structures are called polymorphs. Polymorphs can act as seed crystals, causing subsequent batches of the substance to assume the same crystal structure. A tiny amount of the polymorph can be enough to cause all subsequent batches to take on the same structure. This can pose a problem in the pharmaceutical industry: Once a new polymorph is present in the environment, it can be practically impossible to manufacture the original polymorphs. More here.