A mineral is a naturally occuring, homogeneous, solid with a crystalline atomic structure. Crystallinity implies that a mineral has a definite and limited range of composition, and that the composition is expressible as a chemical formula. Some definitions of minerals give them as inorganic materials, however both diamonds and graphite are considered minerals, and both are primarily comprised of carbon, which would make them organic. So this leads me, as an engineer, to believe that mineralogists do not have a good, precise definition of a mineral, but rather a loose definition. The definition above, is the most inclusive and would include all substances currently described as minerals. The key items that make something a mineral are occurring naturally, and the definite crystal structure, that is expressible as a chemical formula. Rocks that do not meet this criteria are referred to as amorphis - not having a definite structure or expressible as a chemical formula. Some elements that occur naturally and are minerals are arsenic, bismuth, platinum, gold, silver, copper, and sulphur.

THE DEFINITION OF ORGANIC: Organic chemistry is the study of those substances containing carbon in combination with hydrogen (H), and a few other non metals, namely oxygen (O), nitrogen (N), sulfur (S) and the halogens (F2, Cl2, Br2, and I2).

Gold nugget on pyrite. Notice pyrite has flat surfaces and a brassy color, gold has neither.

This is a complex gold ore with oxide gold in a quartz vein with sulfide gold. The sulfide gold is too fine to see, it was probably hydrothermally deposited as a replacement for the iron and pyrites (dark holes) that reduced and oxidized to iron (staining the ore), leaving cavities for gold to enter with the water or steam. (My theory)
Ore is from a California deposit.

Another photo of a gold nugget.

The chemical element gold, atomic number 79, symbol Au (from the Latin aurum), is a soft, lustrous yellow, malleable metal. It is one of the transition metals and its atomic weight is 196.967; it belongs to group 1B in the periodic table along with copper and silver.

Although the Earth's crust averages a mere 0.004 grams of gold per ton, commercial concentrations of gold are found in areas distributed widely over the globe. Gold occurs in association with ores of copper and lead, in quartz veins, in the gravel of stream beds, and with pyrites (iron sulfide). Seawater contains astonishing quantities of gold, but the process of recovery is not economical.

The distribution of gold seems to validate the theory that gold was carried toward the Earth's surface from great depths by geologic activity, perhaps with other metals as a solid solution within molten rock. After this solid solution cooled, its gold content was spread through such a great volume of rock that large fragments were unusual; this theory explains why much of the world's gold is in small, often microscopic particles. The theory also explains why small amounts of gold are widespread in all igneous rocks; they are rarely chemically combined and seldom in quantities rich enough to be called an ore. Because of its poor chemical reactivity, gold was one of the first two or three metals (along with copper and silver) used by humans in these metals' elemental states. Because it is relatively unreactive, it was found uncombined and required no previously developed knowledge of refining. Gold was was probably used in decorative arts before 9000 BC. Even civilizations that developed little or no use of other metals prized gold for its beauty.

If you are looking for gold and can not tell the difference between the second photo, of pyrite on a gold nugget, then you will find lots of pyrite, but no gold. Pyrite is referred to as "Fools Gold", since many a prospector brought home t he shiny Iron Sulfate, and staked claims on their "gold " deposit, which turned out to be pyrite. All that glitters is not gold.