* Composition: A gold nugget is primarily made up of pure gold (elemental gold, Au).

* Uniformity: It has a uniform composition throughout.

* Fixed Properties: Gold nuggets have consistent physical and chemical properties, like density and melting point.

However, it's important to note:

* Impurities: While a gold nugget is primarily gold, it may contain trace amounts of other elements. These are impurities that don't change its fundamental nature as gold.

* Alloys: Some gold nuggets might be classified as alloys if the impurities are significant enough to alter its properties. An alloy is a mixture of metals.

So, while a gold nugget is *primarily* a pure substance, it's possible for it to contain minor impurities or even be an alloy in some cases.