Adularia is frequently a component of the popular gemstone Moonstone .
All moonstones are feldspars. Feldspars are aluminium silicates which contain some proportion of either sodium and potassium, or sodium and calcium. When feldspars cool they are not always able to form a stable mineral with these proportional concentrations. They separate out to form 2 slightly different minerals in occurrence with each other. This exosolution process leads to interesting optical effects. One of the minerals which can separate in exsolution is adularia and this leads to the blue shimmer of the moonstone it is sometimes found in. Not all moonstone contains adularia.
