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If metals are rubbed together, frictional heat will cause the appearance of metal oxides on their surface. G. Beilby mistakenly thought that the initially forming thin surface layer was made of amorphous metal. Later, it was demonstrated by electron microscopy that it was a layer of oxide. It resembles the tarnish caused by oxidation on fresh metals that come in contact with air. See also Tarnish and A morphous alloy.