Separating Fool’s Gold from Gold
Here are a few simple tests that almost anyone can use to tell the difference between pyrite and gold. They can usually be done successfully by inexperienced people. However, wise people obtain a couple small pieces of pyrite and a couple small pieces of gold and use them to gain valuable experience.
CAUTION: All pieces of gold are valuable. However, any piece of gold with a nice crystal habit will have a premium value – often worth several times the value of its contained gold. That premium value can be ruined by some of the tests explained below. So, we have separated the tests into “destructive tests” and “non-destructive tests”. Be careful if you think that you might have a valuable gold specimen.
A) Tarnish: Most specimens of pyrite, found in nature, will have at least some tarnish on their surface. Nuggets or small flakes of gold are usually bright and untarnished.
B) Color: Pyrite has a brassy color. Gold has a golden to yellow color. Most native gold is alloyed with silver, and if the silver content is high enough, the specimen will have a whitish yellow color.
C) Shape: Pyrite is usually found as angular pieces, and many of them exhibit the faces of a cube, octahedron or pyritohedron. Most gold particles found in streams have slightly rounded edges, but be careful – some crystalline gold specimens can display a crystal habit that is similar to pyrite.
D) Striations: Many crystals of pyrite have fine parallel lines on their faces. Gold crystals do not have striations.
E) Specific Gravity: Gold has a specific gravity of about 19.3. The specific gravity of pyrite is about 5. (All gold found in nature is always alloyed with other metals. These metals have a specific gravity which will reduce the specific gravity of the specimen, but never enough that it approaches the specific gravity of pyrite. Specimens containing a significant amount of gold will always have at least two to three times the specific gravity of pyrite.)
A) Streak: Gold has a yellow streak. Pyrite has a greenish black streak. Learn how to do the streak test here.
B) Hardness: Gold has a Mohs hardness of 2.5, while pyrite has a Mohs hardness of 6 to 6.5. Gold will not scratch a copper surface (Mohs hardness of 3), but pyrite will easily scratch copper. Gold can be scratched by a sharp piece of copper, but copper will scratch very few other materials. Learn about the Mohs hardness test here.
C) Ductility: Gold is very ductile, and a tiny piece of gold will bend or dent with pressure from a pin or a pointed piece of wood. Tiny pieces of pyrite will break or resist the pressure.
D) Sectility: Small particles of gold can be cut with a sharp pocket knife. Small particles of pyrite cannot be cut.
Other Minerals That Can Fool You!
Chalcopyrite and small pieces of biotite mica can fool you. Chalcopyrite (a copper iron sulfide) has very similar properties to pyrite. It has a lower hardness than pyrite (3.5 to 4) and a lower specific gravity than pyrite (4.1 to 4.3), but the same tests can separate chalcopyrite from gold. Chalcopyrite also has a greenish black streak.
Many people are surprised that biotite mica can fool people into thinking that it is gold. This most often occurs when an inexperienced person is panning for gold and sees a bright flash in their gold pan. After chasing the tiny, highly lustrous flake, they think that it might be gold. However, slight pressure with a pin can break the flake of mica, but a tiny flake of gold will bend around the pin.
This Post Appeared First On Geology.com