Gold naturally occurs in a solid solution series with the native elements silver as electrum and naturally alloyed with palladium and copper. It sometimes, which is very less common, occurs in minerals as gold compounds, often with tellurium (gold tellurides). It is resistant to most acids, even though it is still said to dissolve in aqua regia, which forms a soluble tetrachloroaurate anion. It is said to be insoluble in nitric acid, which dissolves base metals and silver. This property has been long used to refine gold in metallic objects, hence also known as acid test.