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Quoted from "Crystals and Crystal Growing" (pic related):
>About ten years ago a company was operating a factory which grew large single crystals of ethylenediamine tartrate from solution in water. From this plant it shipped the crystals many miles to another which cut and polished them for industrial use. A year after the factory opened, the crystals in the growing tanks began to grow badly; crystals of something else adhered to them as shown in Plate 11, something which grew even more rapidly. The affliction soon spread to the other factory: the cut and polished crystals acquired the malady on their surfaces.
>Enough of the unwanted material was collected to make a supersaturated solution of it. Since crystals of both materials, the unwanted and the wanted, would grow in that solution, the unwanted substance must contain the desired substance. And since crystals of both would grow in a pure solution made from the desired crystals, the unwanted crystals could not be the result of an impurity which had crept into the solution during the manufacturing process.The wanted material was anhydrous ethylenediamine tartrate, and the unwanted material turned out to be the monohydrate of that substance. During three years of research and development, and another year of manufacture, no seed of the monohydrate had formed. After that they seemed to be everywhere.
One thing chemists and physicists never have explained adequately is how a certain substance just randomly form in the exact same way without any contact with the original contaminated solution. It is especially weird because the desire substance is totally free from water while the defective substance contains water.