A type of ore mined in Nevada
Looks like ordinary rock, but contains 1/10 of an ounce of gold.
They found a few of these deposits in the 1930's
a lot of prospectors didn't recognize it as gold, so wasn't really
discovered for it's value until the 1960's.
If you were to crush it and mine it, which is what old time miners used to do to test their ore,
crush down a sample and process it in a pan to see what their heavy elements were
if you did this with this piece of gold, the pieces would be so microscopic you wouldn't see them in the pan.
So it took geologists years to figure out what this was, once they did they got the hang of it.
Another way they used to test it in the 1800's was to fire it and flux it and collect it into a lead button, then collect the gold out of the lead button. [Fire acid]

This one below has some sulfides in it.
From a volcanic deposit.
Even though this doesn't have a lot of visible gold,
it does have quite. bit of gold.
Very rich, very high grade ore that was mined in Nevada.

This is an unusual type of gold ore.
Mined in 1960's
Underneath was a layer of dark rusty material
under that was some rich copper ore
The mine threw the dark layer away at the dump
because they wanted to get to the copper layer underneath it.
No one bothered to test the dark layer,
it is a Gossan type of material

[Gossan (eiserner hut or eisenhut) is intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral vein.]
gossan, rust-coloured oxide and hydroxide minerals of iron and manganese that cap an ore deposit. > Gossans form by the oxidation of the sulfide minerals in an ore deposit.
basically pyrite or iron.

Pyrite is the most common of sulfide minerals and is widespread in igneous, metamorphic, and sedimentary rocks. It is a common accessory mineral in igneous rocks, where it also occasionally occurs as larger masses arising from an immiscible sulfide phase in the original magma.

Also called fools gold,
eventually after it rusts and decays, decrepitates, it ends up like this,
but the Gold is still in it.

Here is the mineral pyrite.
It looks yellowish and metallic, but is not gold.
It is a mineral that is a combination of iron and sulfur.

Pyrite can have value as a pretty mineral specimen and some can have gold in them.
Sometimes silver and gold Do Occur in fools gold as at times they occur together.
Sometimes there can be very tiny specs of real gold dust inside the pyrite crystal.

Here is some quartz vein with Pyrite

Here is what Pyrite looks like as it oxidizes

This specimen has the oxidized pyrite, but there is also gold in this quartz piece.

This is a Telluride mineral

A telluride mineral is a mineral that has the telluride anion as a main component.
Tellurides are similar to sulfides and are grouped with them in both the Dana and Strunz mineral classification systems.

In addition to solar cells, tellurium in the form of semiconducting bismuth telluride is used in thermoelectric cooling devices for military and electronics applications, such as the cooling of infrared detectors, integrated circuits, laser diodes and medical instruments.

calaverite, a gold telluride mineral (AuTe2) that is a member of the krennerite group of sulfides and perhaps a structurally altered form (paramorph) of krennerite (q.v.); it generally contains some silver replacing gold.

Here is a Telluride mixed with gold
this one has fooled miners before as they will think it is just some sulfur mineral, but this one has gold in it.
This group contains about half gold by weight in the calaverite. There is some silver in these pieces also.

calaverite, a gold telluride mineral (AuTe2) that is a member of the krennerite group of sulfides and perhaps a structurally altered form (paramorph) of krennerite (q.v.); it generally contains some silver replacing gold.

Calaverite in ore from the Triple Creek District in Colorado

The miners learned that this calaverite is a natural combination of gold and tellurium
and the gold can easily be extracted from it.

This is from a mine in Montana.
It has a little bit of telluride in it.