Usually one speaks about isomorphous substitution. One element can substitute another if its atomic size is similar. For ions, the situation is similar, but the charge must be also the same. Substitution is also possible if the charges are different, but a charge compensation ion must be also introduced. For compounds composed of several elements or ions, a few or all of them could be substituted. If all of the elements (ions) of one compound can be substituted by those of another compound one usually speaks about solid solutions. In this case, the crystal structures (best defined by their space groups) must also be the same besides ionic sizes and charges. There are a lot of possible combinations of substances which form solid solutions. Take for example aluminium oxide (the mineral corundum) and chromium oxide (the mineral eskolaite). Both have the same structure and chromium and aluminum have the same charge (3). The chromium oxide which is green substitutes isomorphically the alumina forming a pink (reddish or purplish) solid solution which is a rear mineral called Ruby. There are many other minerals which form solid solutions (e.g. garnets, spinels), many metals, soluble salts, etc.