As long ago as 1890, Hiorns observed as follows:
:"Noble Metals. Gold, Platinum, Silver, and a few  rare metals. The members of this class have little or no tendency to unite with oxygen in the free state, and when placed in water at a red heat do not alter its composition. The oxides are readily decomposed by heat in consequence of the feeble affinity between the metal and oxygen."
Smith, writing in 1946, continued the theme:
:"There is no sharp dividing line [between noble metals and base metals] but perhaps the best definition of a noble metal is a metal whose oxide is easily decomposed at a temperature below a red heat."
:"It follows from this that noble metals...have little attraction for oxygen and are consequently not oxidised or discoloured at moderate temperatures."
Such nobility is mainly associated with the relatively high electronegativity values of the noble metals, resulting in only weakly polar covalent bonding with oxygen. The table lists the melting points of the oxides of the noble metals, and for some of those of the non-noble metals, for the elements in their most stable oxidation states.