Gold is one of a small number of minerals that is a single chemical element occurring naturally in so-called 'native' form, i.e. uncombined with oxygen or other elements. Gold crystallizes in the isometric (cubic) system and is found in cubes, octahedra and other forms with isometric symmetry. Excellent crystallized gold occurs in California, and good specimens have been found in Placer County. Gold also occurs in veins and scattered particles, and in stream beds as nuggets. It is a 'noble' metal, which means it is not chemically reactive and therefore is generally bright and lustrous. Its color, luster and great density have made gold desirable to humans throughout history.