In nature, gold most often occurs in its native state (that is,
as a metal), though usually alloyed with silver. Native gold
contains usually eight to ten percent silver, but often much more -
alloys with a silver content over 20% are called electrum. As the
amount of silver increases, the color becomes whiter and the
specific gravity becomes lower. Ores bearing native gold consist of
grains or microscopic particles of metallic gold embedded in rock,
often in association with veins of quartz or sulfide minerals like
pyrite. These are called "lode" deposits. Native gold is also found
in the form of free flakes, grains or larger nuggets that have been
eroded from rocks and end up in alluvial deposits (called placer
deposits). Such free gold is always richer at the surface of
gold-bearing veins owing to the oxidation of accompanying minerals
followed by weathering, and washing of the dust into streams and
rivers, where it collects and can be welded by water action to form
nuggets. Gold sometimes occurs combined with tellurium as the
minerals calaverite, krennerite, nagyagite, petzite and sylvanite,
and as the rare bismuthide maldonite (Au2Bi) and antimonide
aurostibite (AuSb2). Gold also occurs in rare alloys with copper,
lead, and mercury: the minerals auricupride (Cu3Au), novodneprite
(AuPb3) and weishanite ((Au,Ag)3Hg2).
